Alright, I looked at this a bit. Here are my thoughts:
An Analysis of Longevity in Ars Magica Fifth Edition
The creation of [[Longevity Ritual|Longevity Rituals]] is detailed in the core book (page 101). There it says that “magi of two hundred years or older are rare, but not unheard ofâ€. This analysis purports to see just how old can magi get by looking in depth at the game mechanics.
== Inventing the Ritual: Assumptions on Specialists
There are two separate processes involved in Longevity Rituals. The first is the creation of the initial focus, which is based on a Creo Corpus Lab Total; often that of a specialist assisted by the commissioning magus. This initial stage is a Seasonal activity that requires 1 pawn per five years of age (rounded up). As an option, you can choose to invest more raw vis, adding +1 to your Lab Total per pawn. The ritual provides a +1 bonus per 5 points of the Lab Total (rounded up).
As a Seasonal Activity, the amount of raw vis you can use in limited by Magic Theory x2 (page 94). On these grounds, the best strategy is therefore to commission a longevity ritual from the best specialist at the youngest possible age. At the age of 35, 6 pawns would be required due to age. Assuming, for example, that the specialist has a Magic Theory of 15 this allows him to add +24 to his Lab Total by investing 24 more pawns of raw vis. Politically, this is very difficult to arrange by may be possible for a magus supported by a cohesive mystic society, such as a member of House Tremere or a tight-knit mystery cult.
The specialist can make use of Leadership assistants in his work (page 103). Assuming a (modest) score of 3 (or 2 with an appropriate specialty), he can therefore make use of four assistants – his own familiar, the commissioner, and either the commissioner's familiar or his own apprentice. Here, there is an advantage to creating rituals for elder magi – they would have a higher Magic Theory score and a well-developed familiar. This is contrast to the earlier paragraph, that recommended a younger commissioner. Since the bonuses more or less balance out, we will focus on the scenario of a young commissioner, but bare in mind that an elder magus and his familiar might actually provide a better set of circumstances, depending on their Magic Theory and Intelligence scores.
The Lab Total is clearly dependent on the specialists' stats. The core posits (on page 32) that the highest Art scores are 10 plus one for every four years since apprenticeship, and that “very few†magi are much older so that the highest Art scores are 40. This is based on 30 XP and 2 WP per year.
Using this is a basis to work from, we can assume both Creo and Corpus at 40. This requires 1640 XP, which are about 43% of the magus experience points – a significant but reasonable investment, allowing room for other pursuits and Abilities. Note that this requires 82 Tractatus, each with Quality 10, in each of these Arts (or some alternative Sources); this is very considerable, and probably takes up most – if not all – of the payment for the specialist's work.
Another key issue is the Magic Theory score. Our specialist still has plenty of experience points left, so we shall invest 600 (another 16%) in obtaining a Magic Theory score of 15.
Does he have any pertinent Virtues? There are many Virtues that can affect the Lab Total, including Minor (or Major) Magical Focus, Cyclic Magic, Inventive Genius, Puissant Art, and Great Intelligence. For the purpose of this analysis, we will assume only a pertinent Magical Focus. Note that the other virtues may contribute as much as +14 to the Lab Total.
The specialist's Intelligence score can therefore not be higher than +3. As this seems a reasonable Intelligence score for magi in the canon, we'll assume it.
A last consideration is the Aura. The specialist is likely to reside in a highly magical locale, perhaps even one with an aura attuned to Corpus (see ''Realms of Power: Magic''). To be somewhat conservative, we shall assume a Magical aura of 6.
These are all the pertinent stats for the specialist, but what about his familiar? As per page 105, the familiar is likely to have an Intelligence score of -3, assuming it started as a Cunning creature. It can learn Magic Theory and serve as a laboratory assistant, and indeed “[o]ver the years, your familiar learns what you knowâ€. It is therefore likely to have the same Magic Theory score as the magus, or 15 in this case.
What about the specialist's laboratory? The rules in ''Covenants'' allow to specialize and customize the laboratory. Depending on how much time and effort the magus spends on it, he can considerably increase his Lab Total. Let us assume a relatively modest bonus of +6. This can be achieved rather swiftly, perhaps with a year's work (30 XP). Note that if you do not use the ''Covenants'' supplement, these points might be obtained from the various options we neglected above, such as piling on extra Virtues, a higher Aura, or so on.
Finally, there is the matter of the statistics of the other assistants. In our young-commissioner scenario, these consist of the specialist's apprentice and a young magus. For the sake of simplicity, we shall assume both have Intelligence +3 and Magic Theory 5.
As a final note on the statistics, consider the use of magic to increase Characteristics. If raw vis is plentiful, such rituals can increase the Intelligence of all the participants to +5. We shall ignore this option in this analysis.
To summarize, our scenario involves a highly-skilled, 120-year old specialist working in a powerful aura to fashion a longevity ritual for a young commissioner, assisted by his familiar and apprentice. The resulting Lab Total is 202 (3 Int+ 15 Magic Theory+ 40 Creo+ 40 Corpus+ 40 Focus+ 6 aura+ 24 raw vis+ 6 lab -3 familiar Int +15 familiar Magic Theory +3 apprentice Int +5 apprentice Magic Theory +3 commissioner Int +5 commissioner Magic Theory). This ritual requires 30
pawns of raw vis, and provides a +41 bonus to aging rolls.
== Living Under the Ritual
Warping points accumulate under the longevity ritual. You accumulate 1 WP per year due to the effect of the Longevity Ritual itself (page 168). For magi, this has no effect except rendering the magus more susceptible to fail to avoid or control Twilight episodes. His Warping score is therefore irrelevant, as long as he is careful not to engage in activities that might result in a double-botch or otherwise expose him to Twilight.
At the end of each year, the magus must make his aging roll (page 169). To avoid aging completely, the roll must be 9 or less. Assuming +1 Living Conditions (a minimum for a magus), this means that the die result must be smaller than (33-age/10). The ritual will only fail more rarely, however, at either a roll of 13 or 22 or more.
Simulation indicates that this yields a lifespan of 440 to 480, with 3 to 4 crises and recreation of the ritual, and the first crises occurring around age 350. This is considerably more than the lifespan of about 145 years that the above analysis was based on!
We can see that there is an excess of power, allowing magi to live longer than the setting assumes if only they pursue powerful longevity rituals. The degree of excess is considerable, so that even tapping it partially will allow magi to reach great age.
== Recreating the Ritual
The second possible process is repeating the ritual, recreating the focus from the Lab Text. This involves the same amount of raw vis (based on your current age), but no significant investment in time. As this is not a seasonal activity, the magus is limited by his Art scores in the amount of raw vis (page 82). This means 70 pawns at age 350, and as it is split between two Arts this should be manageable even by a non-specialist magus for the most part. Even increasing age and adding he raw vis invested on top of the age (24 pawns in the example above) is still easily within reach of magi, requiring at best access to young or only partially specialized magi.
=== Conclusion
Assuming a few magi specializing in this in the Order, it seems creating very powerful longevity rituals is easy. Actually using them requires something like 30 pawns of raw vis, increasing with age to perhaps 60 pawns. These are considerable sums, but far from excessive in most sagas. Such rituals can drastically extend the magus' lifespan, to 460 years or so, and are unlikely to fail before the age of 350. These extreme ages in turn will lead to greater power and specialization. Even assuming one Warping Point per year, these magi will have a Warping score in excess of 10 so that they must take every measure to avoid accidentally falling into Twilight.
Such extremely old and powerful magi are not conductive to the setting. First, their higher Arts will increase the available Summa levels, and the hunger of longevity specialists for high-Quality sources will lead to huge libraries of tractatus on at least Creo, Corpus, and Magic Theory. Secondly, such great ages hamper the progress of history, as a 460 year old magus has lives through much of it. Thirdly, the great Art scores mean that there is a need to develop more powerful enemies and more powerful spells, reducing the effectiveness of those presented in the setting. Furthermore, it encourages overly long sagas, whereas most sagas span too short a time in comparison with character advancement as it is. Finally, the prevalence of older magi increases the dichotomy between the player characters' power and that of the common magus in their Tribunal, taking some shine off the PCs.
I therefore recommend to completely ignore this analysis. Pretend that that core book makes sense, so that indeed most magi live to be 145 years old or so and succumb to Twilight soon after. Let the player characters, however, revel in their power. If one of them wants to create a powerful longevity potion, he could probably do very well. Let him. Make stories out of this, if you can. Don't let this be the case for NPC magi, however – keep even longevity NPC specialists confined to relatively low Lab Totals, just enough to allow a 145 year lifespan. For NPCs, anything beyond that is superfluous as they will succumb to Twilight around that time anyway.
You know, I still have my ArsM 2nd ed rulebook around here somewhere.
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