Almost instantly. I've never seen any take time to kick in, so they should be very useful if you're intending to be in the front of the fight rather than defending.

[ She looks over at her potions, checking them - all of them seem like they'd be useful over the course of an actual fight, if they're swallowed quickly enough and not ignored. Pausing, she considers it before she nods, looking at Clarisse with a little more determination. ]

I can bring them over for you while I do the rounds.