it’s worth mentioning there are many variables.
the brand: gandini powerfoams are by far the best. period!i’ve seen jamie tear a set of other brand foams in minutes. once, ph2t wore through a set of squat foams before the batts ran down! i’m not even gonna bother with the crap foams that came with my iwaver!
surface: personally i’ve been running my gandini’s on rough concrete, foams rear and rubber front. my foams last just as long as rubber tyres did. i estimate between 1 – 1 1/2 hours of solid hard racing. like ph2t said, on your surface they should go a long way.