I don't think you could live forever with just (vanilla) Longevity Rituals, even barring Twilight and warping.
Your Longevity Ritual requires the expenditure of 1 pawn of vis for every five years of age, and you are still limited to using twice your [Magic Theory Score] pawns of vis for it. So I think sooner or later you won't be able to spend the vis you need to create the ritual. And even though your Logenvity Ritual can last for a long time, the aging roll is a stress one (with not botch dice). So at some point you will roll enough 1's that you suffer an aging crisis, and thus cancel your Longevity Ritual. Then you will need a new one, and that's when you'll have to worry about your vis usage limit.
At 200 years old, you'll need to spend 40 pawns of vis, which means a score in Magic Theory of 20, or 1,050 experience points. That means getting about 5.76 experience points in MT every year for your whole hermetic life, assuming you came out of apprenticeship at 23 and already had an MT score of 3 by then. Doesn't seem too difficult, now does it? But it's highly improbable you'll find Summae higher than, let's say, maybe level 8. So from that point on, you'll need to read Tractati, which in this case means around 87 tractati (I'm assuming quality 10 for them).
But we're not talking about living up to 200. We're talking about living forever. True immortality.
By the time you hit 700 years old, you'll need a score of Magic Theory of 70, or 12,425 experience points. That means 18 xp towards MT all your hermetic life, straight out of Gauntlet. And considering no Summae over 8 for MT, it also means 1,225 Tractati.
Even though the number of experience points you need to have spent every year to reach the required Magic Theory doesn't grow too quickly, that assumes you retroactively change your assumptions as to what you did in your early hermetic years, which is highly unrealistic. So a more realistic scenario would be you having to increase every year the number of new Tractati you will need to read to keep up with your Magic Theory, so it's high enough when the time comes around to make a new Longevity Ritual.
Soon after 200 years old you'll be needing to read 2 new Tractati every year, which means there being enough unread Tractati around, and having to locate them. Your Magic Theory library now has 87 Tractati.
Soon after 600 years old you'll be needing 4 new Tractati per year. Which means you'll be spending all four seasons every year reading them, since you not only need to have them, but also to read them. I'm not too sure how you'll find the time to locate them without being able to take a single free season, but you could always hire someone to do it for you. Though I'm not too sure what you'd offer in return, since you have no more spare time to do anything for them. Your Magic Theory library now has 928 Tractati.
But still doable.
By the time you pass 800 years old, you'll need to read 5 new Tractati per year. And you only have four seasons. So there's no way you'll be able to keep increasing your current score of Magic Theory 81 beyond that point fast enough that you can actually spend the vis to make your new Longevity Ritual when you have your next aging crisis.
And then you die. Game over.
But you left behind a library with over 1,600 Tractati on Magic Theory. Congratulations.
And that's assuming you only have to worry about Magic Theory, and plenty other circumstances that will never actually happen (unlimited resources, unlimited vis, no strings attached to your lab activities, no distractions, etc). So, in sum, even in the best possible situation, with the whole Order working non-stop to help you live forever, you just cannot live forever just with (vanilla) Longevity Rituals.