i wouldn't apply it if you'll be sweating heavily within 30-60 mins of applying it. it'll still work, but see below. sweat might actually increase minoxidil absorption too.. it can be a good or bad thing.
the minoxidil absorbs pretty quick and you'll still get it into your scalp within that time, but some of the minoxidil will mix with the sweat and drip into your face, etc.. might get some irritation or hair growing where you don't want it..