This seems like an esoteric question... and it is. But I swear it matters if you have a Jewish maga in a game, and are wondering if she has to go to the mikvah.
The longevity ritual renders you sterile. But does that mean that women do not get their menstrual period, or do they get it and just can't get pregnant? My instinct would be that they no longer get their period, but I'm wondering if this has ever come up before.
It depends on how much story you want to make out of the mikvah. If you still want to tell the story about going to the mikvah after she starts with the longevity ritual, you do not break anything by assuming that she has the period. If you think you have told that story many enough times when she was younger, it does not matter if you assume that she lost the period to the LR or to early menopause, or if you simply do not mention it.
Whatever you decide it is plausible enough for a story, and since magic is inherently unpredictable, you do not even have to be consistent.
I'd give the standard answer of "do what is best for the story". That being said...
Part of why women get periods according to scholars of ME is to expel waste matter. There are other reasons, but they're only relevant during pregnancy- hence, imo a maga would still get periods if only to expel this waste.
I've written up many spells and magic items for midwives (actually, most intended for horses at a covenant breeding exotic equines, but they generally apply) and it genuinely didn't occur to me to consider it. Bravo, I now have some more ideas to consider in idle moments.
I'd personally rule that they do not, the longevity potions freeze a number of processes to retard aging, but as Dalt said above, "do what is best for the story". Go crazy, argue with the rabbis whether teleported or created water satisfied the requirements, have fun.
And I really should submit some of those spells and items during November one of these years...
I think the most "realistic" answer is that it varies from maga to maga, depending on how they design their Longevity Ritual. Conveniently, that also maximises story options!
To elaborate on this, the medieval medical explanation for periods, based on Galen, is that women are constitutionally more "cold" than men. This lack of heat causes them to "cook" nutrients in their body less efficiently, producing more waste. In addition, women have less capacity for expelling waste through sweat and growth of body hair. This requires them to have an extra way of expelling waste. During pregnancy and after, the excess matter goes to the child or to produce milk, which is why periods stop. So according to medical authorities, periods have nothing to do with reproduction or expending life energy, so longevity rituals would not prevent them. Since periods were seen as a sign of health in a women, there is an argument to be made that longevity rituals prevent menopause.
On the other hand, Hildegard von Bingen links periods and fertility in Causae and Curae: ‘‘The stream of the menstrual period in woman is her generative greenness and floridity, which sprouts forth offspring; for just as a tree flowers in its floridity and sends forth branches and produces fruit, so the female extrudes flowers from the viridity of the streams of menstrual blood and produces branches in the fruit of her womb. But just as a tree which lacks viridity is said to be unfruitful, so, too, the woman who does not have the viridity of her flowering at the proper age is called infertile.’’ This links periods to fertility pretty strongly.
So yes, in conclusion, do what is best for the story, though since there's arguments for both options and longevity rituals are highly personal, it's easy to justify the effects varying from person to person.