If by some strange chance several PC's had participated in a strange none hermetic ritual which had caused their longevity potions to fail would they have noticed that the potions where no longer in effect , or would it be at their enxt aging roll?
It would depend on the Longevity ritual they made. The details of what it actually is are deliberately left vague, so in theory I suppose someone's Ritual might have a visual component which allows them to see the efficacy of their ritual; like a tattoo that fades as it loses effectiveness. Others that are paranoid might have spells prepared that they cast in themselves every once in awhile to check themselves.
But I'd say the average ritual probably doesn't have an alarm or indicator that the Ritual has failed. They might have to wait to their next aging roll.