Sure it is possible and not even that difficult.
Probably (hopefully!) not for a magus straight out of Gauntlet, but after a few years?
Easy!*
- [size=85]Disclaimer:[/size] [size=50]May or may not actually be easy depending on local conditions. YSMV.[/size]
By the time a verditius needs a longevity ritual, he/she should be able to reach MT10 at least. The Order has LOTS of books on that particular subject.
Well, a few years? Maybe not. Unless few is 15-30 years from gauntlet. And easy is true, all advancement in Ars is easy, it's presumed in the setting. A lot of times, though, magi can't pursue their hearts desire to the exclusion of everything else. Stories come up, whether as a result of story/personality flaws, desires of the player, or covenant hooks that pull in the character. So, I will agree that it's easy, but your definition of few conflicts with my definition of a few: 4 or 5. 
Well, of course one might be able to pursue little else but Magic Theory and Philosophiae in their studies, where their scores form a product of 60 or greater. Assuming no virtues to augment the aforementioned abilities and both abilities start at a score of 3, with an average source quality of 10 for advancement purposes, it will take 16 seasons to get a score of 6 and 25 seasons to get a score of 10; just a bit over 10 years in total. As you said in the fine print, which I did not include, YSMV, but few magi can ever dedicate themselves to such a pursuit, something always gets in the way. It's probably reasonable to double the time to 20.5 years. Then we have the issue of creating the 45th level ritual spell, too, which requires a lab total in excess of 45 (not hard) and the time to devote to inventing the spell. I generally prefer to create specialists, but few specialists I've made have been capable of creating 45th level spells in reasonable time frames at gauntlet. Even getting to a lab total of 50, makes creating the spell take just over two years, again double it for life getting in the way type things.
So, I could see a magus more than 30 years past gauntlet capable of doing this, virtues could shave off a fair bit of time, but close off other capabilities of the magus. It might also be true that higher source qualities that 10 are possible, again reducing the time. 30 years seems about right to me, though in creating a character that can do something useful with a 57.5 kg (~125 pounds for those of us in the US) diamond, besides just opening it for enchantment. Something that negates the weight of the diamond would be pretty important, IMO.
The logistics behind creating the ritual for such a large diamond, coupled with the logistics necessary to having a high score in Magic Theory and Philosophiae don't suggest a few years to me. A particularly obsessed/diligent magus might well get this done in 30 years, less if he's singularly gifted in these areas, but still something that will take more than a few years.
You are making a rather large assumption about all Verditius magi. Starting with an affinity and a starting score of 3, and an average SQ of 10, that's 17 seasons. If you add Puissant to the mix you can get it down to 12. If a magus starts at 25, he's got 40 total seasons before needing a LR, with boosting MT taking up a good portion of his time, over 1/4 to nearly 1/2, depending upon his natural gifts (virtues). If he has neither affinity or puissant magic theory, he's looking at 25 seasons, or over half of his time devoted to improving Magic Theory. While Magic Theory is important, it's more important to making stuff when one can, when one is a Verditius.
I don't disagree that the Order has lots of books on the subject, and even if you make them easily available to the character, it will still take a lot of time to just advance the score.
If you assume a SQ of 10. A book of level 6 and quality 17 is not unreasonable, followed by studying with SQ around 11 whether a level 8 book or sound tracti.
If I assume SQ 10, then what? I'm just taking 10 as the baseline, which is what RAW does.
Sure, there might be a L6Q17 summa out there, but that's a summa that's likely been clarified and is sitting at the covenant of the author[sup]1[/sup], who requires payment to be able to read it. What if his payment for a season of being allowed to read the book is a payment of time? You've now reduced the effective SQ of a Q17 summa to 8.5/season, or he's generous and allows 2 seasons of reading for 1 season of time, you're at 11.33 xp/season. SQ 10 is a really good baseline...
[sup]1[/sup] An author who has Magic Theory 12[sup]2[/sup], Com 5, Good Teacher, employs all the skilled professionals and spent vis[sup]3[/sup] to ensure the book had an additional point of quality. Also needs to have safeguards that the book never leaves the aura.
[sup]2[/sup] Or his MT is 14, and he doesn't clarify it, but it still represents a huge investment in time. This opus isn't going to be freely and easily distributed throughout the Order, is it?
[sup]3[/sup]It's at least 8 pawns of vis, but it could be 16 if opened as a compound device, if a magus is doing this, in for a penny, in for a pound to enchant the book as well as having it clarified.
I'm reasonably certain I included a disclaimer with the above statement.
I don't read fine print. Sagas might vary, but my estimates will probably hold for a majority of the sagas out there.
Yes, 60 is quite doable for magi not that long after gauntlet if they have the Verditius Elder Runes Virtue. The above talk seems to focus on Magic Theory. Don't forget that there are enough summas and tractatus in the mundane world on Philosophiae to get really far, and Philosophiae is really useful to Verditius magi both for lab work and for ceremonial casting. Plus keeping both around the same point costs less experience. Also, if the Verditius has an appropriate Craft, the necessary amount drops.
E.g. Let's say we have a Verditius who starts with Craft: Gemcutting (Verditius Magic) 3, Magic Theory (Items) 3, and Philosophiae (Verditius Magic) 2. I think it can be agreed that this magus has not been overly geared toward the goal at the start, though maybe a touch with the Craft. This magus can reach the goal with Magic Theory 7 and Philosophiae 6 because (7+1)x(6+1)+4=60. That's 110 experience in Magic Theory and 90 experience in Philosophiae, and mostly reachable via summas. Sure, that takes a little dedication, but these are also probably the two most valuable Abilities for most Verditius magi. Also, those targets could be lower if Puissant factors in or easier to reach if Affinity/Book Learner/etc. factor in. It's quite doable only a few years out of gauntlet, especially with a useful Virtue. It's easily doable within 10 years without much focus if desired. I would expect nearly every Verditius like this would be there by 20 to 30 years out of gauntlet.