Limit on vis per season is MT * 2.

Longevity ritual is 1 pawn per five years of age (the age of the subject at the time the LR was created).

If you have a MT of 8 (which is pretty friggin high) and specialized in longevity rituals, so effectively a score of 9;

9 * 2 = 18 * 5 = 90 years old is the max age for this LR.

This was discussed on David Charts list and the small number of replies agreed that this limit of MT * 2 for vis is ok. There were also discussions on how difficult/easy it would be to raise your MT to 20.

My question is: do you all think that this restriction on vis to MT * 2 is appropriate for LR, or can an exception be made to this rule? And where would adding extra vis to the potion come into play?

Considering that only the developing of the LR takes the whole season, and making a new one (using the old formula) only a little enough time not to interfere with laboratory work, I would say that the 2 x MT limit is not really appropriate.

However, I would rather use the limit you have on spells, so you can only use as many pawns of vis as you have score in the Art (of the vis used).

my take is as follows:

the 2*MT limit applies during the season-long research to devise a new formula; however it is common for junior magi to receive assistance in this from their parens, or even (as they get a bit older) to commission one from a specialist.

In 5e, you use the existing dose until you get an aging crisis at which point you need only create a new dose, in "almost no time" - I have interpreted the latter as meaning that you create a new dose of the longevity ritual using the rules for ritual magic (Creo + Corpus total for limit fo pawns of vis) so even a junior magus can rebrew the formula devised for them by a specialist.

You can carry on rebrewing the same formula as long as you desire... You may eventually decide that you need a formula from a specialist to alleviate increased age - but the specialist's formula is likely to have the potential to prtoect you from aging into the 200's or 300's.

We note at this point that the actual limit on the lifespan of longevity assisted magi is Warping & Twilight - you accumulate 1 point per year just from the longevity effcts, plus any other long term of powerful spells, plus botches.
Twilight will take you before old age...

this may be worth posting here :

"However, you might want to revise the limit on seasonal vis usage up, to
say triple Magic Theory. The current limit is a real, serious limit, and
loosening it a bit may better suit some sagas."

--
David Chart
Ars Magica Line Editor

"Twilight will take you before old age..."

while this is likely to be true I'll note that in fifth edition twilight won't take you until you have a twilight experience that you don't recover from. And while it is true that after a certain amount of twilight is gained it is impossible for you to recover from a twilight experience, a character can put off that last twilight experienc for quite some time.

"Temporary" twilights apparently only occur on double botches or worse in the present rules. With a strong (3+) golden cord and frequent use of mastered spells it becomes very uncommon for a magus to ever roll more than one botch die.

If you're sufficently careful and well prepaired age can get you before twilight.