In RoP; Magic it says magical characters don't need to eat, sleep, breath. Which really means if you have a magi that transforms he can double his amount of lab work. The down side is a dark secret which can get you killed since the code no longer applies.
If this is correct it seems a major flaw in the system...Theoretically you can get accelerated study for awhile then get your "magicness" removed via PeVi and be in the clear.
You... can? I'm not sure you can revert back to being mortal like that.
At any rate, immortal characters break the game. It's entire premise is that power (experience points, spells...) come with time (spent studying), and immortal magi do so indefinitely so are arbitrarily strong. The limiting factor for magical creatures seems to be raw vis, not time; and I don't thing it's too much of a factor, frankly.
But yes, not needing to sleep is generally a great benefit. Not sure why it's so, though - certainly there are legends of sleeping giants, dragons, and so on...
Maybe all that not eating, sleeping, etc. finally catches up with them. 
No, it doesn't break the system if the SG is on top of things. Immortal magi are, in theory, limited in their ability to learn more than they knew at the point of their transformation. Whatever vis they would normally use on enchantments, rituals, studying, etc instead goes to just letting them learn stuff.
Regarding PCs specifically......... If the magus starts the game immortal, that's going to require buying a bunch of virtues to pay for that. If the PC becomes immortal in the course of the game, then its up to the SG to make sure that sufficient effort went into doing so to account for the benefits. Most of the 'standard' ways to become immortal are very difficult, take a long time to master, and draw away valuable resources (time/vis/etc) from the process of getting better at the Hermetic Arts. Other story based ways should be equally 'expensive'.
More importantly for game play, the immortal magus PC isn't actually playing the game any longer than the other PCs so its not a balance issue there. He might get some more study time, but at considerable cost.
The "Need No Sleep" major virtue in older editions actually added 1 season per year to effective study time, fyi.
Lab work is tied to the seasons, so it's limited to one lab activity each season. Characters can't stretch that. You might be able to justify a small bonus to the character's Lab Total for following an unusual schedule (I think that's in Covenants).
Some characters who don't need to sleep do get additional experience in Fifth Edition books, mentioned in RoP: The Divine and in HoH: Mystery Cults I think. It gives them two more seasons (for study only) each year. However, magic beings don't get this mechanical benefit, because their potential for learning is also tied to seasons: they need vis to make their experience points permanent, and the amount of vis they can spend each season is limited. So for them, any and all learning is a seasonal activity whether they do it 24 hours a day or not.
True. Or at least, many magi agree that when you become a magic being, you are no longer a magus, because otherwise ghosts would rule the Order.
I don't think PeVi can remove the Virtue that makes the character a magic being. However, accumulated acclimation can destroy the character's magical nature. What happens then varies, however. For my saga, I usually rule it turns back into a mundane version of its true form, which would mean a Magic Human magus would become human again, probably without The Gift.
I'm not sure how immortal magi would stand with the rest of the order. I suspect there would be a strong resistance to having them marched. Bjornaer magi can become great beasts, and they wouldn't stand by and let their ancestors be hunted down. Merinita have the mystery of becoming, and once had a prima who had become a faerie. The rhine tribunal had a decision swayed by the votes of magi's ghosts.
There's a strong precedence for allowing magical/immortal magi to be allowed to stay in the order.
Actually it says you won't die for lack of those, but that they still suffer if they lack them: see RoP:M p.29 col 2 "however, they cannot benefit from this, because in conditions that would cause a character without Magic Might to suffer from deprivation or exposure, magic characters do not recover Might Points or Fatigue levels, and their wounds have an immediate chance of worsening"...
I would go further and point out that prolonged work (eg lab work) is tiring, and all magi accumulate some faitgue while working, but they "rest it off". The immortal over-worker will tire, but not "rest it off" unless they rest - stick with the spirit of "they cannot benefit from this"!
By a normal magi using a lengthened schedual they get more done but acumulate fatigue and can take a hit on aging rolls since they don't get their normal sleep. I find it a BIG stretch to think that the time it takes you to read a book is determined by the planets rather than time spent actually reading.
they subtract their might from the amount of experience they receive, I've seen nothing mentioned about having to use vis to make it permanent in RoP: Magic
I beleive it's on the middel column under the picture on the 1st page of magical characters chapter. A magical creature who looses all it's might becomes mundane, if losing all it's might won't kill it. So ghost die, people/creatures become a mundane version of themselves
Are they still in the order or just protected by factions of the order?
It's an abstract system, but that's how study generally works. It's like how adventure experience comes at the end of the season, not at the end of the adventure, because it is associated with time turning over what the character has learned under pressure. Learning happens over time.
D'o! Mea culpa, you are correct. I don't know what I was thinking, maybe my mind was wandering to how magic creatures can study from vis, or how they can reduce the penalty to their experience total with vis.
As Neil noted, it's not true lack of sleep, you just don't die from it. In any case, magic creatures were not designed to get extra study seasons as part of their package. I think I agree with you that this would make them too powerful.
It also says they could die; what happens when you lose all your Might is up to the storyguide. Like I said, I probably wouldn't kill a magic human that lost all its power, unless perhaps it had been magic for longer than it had been human.
So you are saying that it is a graduated learning curve? Ie That twice as much study yields less than twice as much learning?
I can see that happening for a 'mortal', but a magical creature doesn't have the same 'frailties" that a person would (they have a different set). It seems illogical and artificial.
No, I'm saying that the amount of time it takes a character to finish a book doesn't change how much experience that character gains in a season. The experience points are gained at the end of the season, not when the character is finished reading the book, because experience points are associated with time passing as much as they are associated with activities. If a mundane character speed-reads two books in one season, he doesn't get experience from both of them, right?
As I said, gaining experience points by season is an abstracted system from the core rules. That's just the way experience is earned. If it helps, you can say that the character is spending the rest of the season thinking about what it read.
Also, just because a magic creature doesn't need sleep doesn't mean it doesn't sleep anyway, unless there is some pressing need for it to stay awake (like keeping watch). I imagine creatures like dragons or sentient objects spend a lot of time not doing anything, effectively sleeping. Part of the character's attention is dedicated to soaking up radiant vis to prevent Acclimation, perhaps?
I'd have their real age catch up to them. 
IIRC, there's a "meditation" ability (or something like that) that works just that way.