If we look at an 'average' hermetic magus of 100 years of age - with a working lifespan of 70 years to this point (maybe more) - we can see that the 275xp required to reach Magic Theory 10 is only 4xp per year on average. Or 2 seasons a year in the lab getting Exposure. Reading Tractatus will generally get you 11-12xp (based on 'sound books' per Covenants), so if you're not doing lab work or taking Exposure elsewhere it's about 1 tractatus of reading per 3 years.
So it's not hard for a magus to keep up with his Magic Theory enough to design his own Longevity Ritual at 100 years of age.
Magic Theory 15, however, is 600xp. At 150 years of age that's 120 years of being awesome and you need an average of 5xp in Magic Theory a year. It's still achievable with a bit of effort, or if you've got Affinity with Magic Theory, which I'd expect any prospective Lab Rat to have. Note that the average magus of that age will have a Warping Score of 9 (240xp) and is probably more worried about Final Twilight than dying of old age. So we can see that the average hermetic lifespan isn't much more than 150 years due to Twilight , but there are Flawless Magic/Cautious Sorcerers/+4 Gold Cord Score magi out there who can last a lot longer! Magi at this point want a Lab Total of at least 75, which is still within range of non-specialists with good aura/lab/apprentice/familiar bonuses, though you'll probably want to seek out a specialist regardless.
Magic Theory 20 takes 1050xp over a 170 year span, that's 6.17xp per year. For the Affinity with Magic Theory lab rat (~2 seasons a year in the lab taking Exposure) this is still achievable. Even the non-lab rat can manage it by reading a tractatus every other year or taking sabbaticals of book reading every decade or so. Unless he's a LR specialist himself, however, the +20 to Aging rolls from his age are pretty hard to top as he'll need a Lab Total of 100+ just to stay in the 'increase apparent aging' zone - great if you have the Unaging Virtue, not great if you don't want to appear to be a withered old wreck. Certainly any magus could achieve Cr40 Co40 if they spent all their days chasing it, but then you've spent all your time and effort just to live longer and you might have better spent your time chasing Mythic Alchemy to get a Greater Elixir or other sort of actual immortality.
So it's entirely reasonable for magi to stay ahead of the Magic Theory requirements of their own Longevity Ritual, but staying ahead of the Lab Totals pretty much requires a specialist (with Focus and Affinities in Creo and Corpus). That magus CAN keep ahead of all the requirements and will be in much demand from magi of all ages.