20 seconds is a lot longer than it sounds, especially in the middle of a big battle. Not that it'll matter most of the time regardless, since after they take the first punch they're either going to be a messy smear on the ground or beefy enough to remain unharmed but likely in the next county over. Since he's a trained fighter he should be throwing blows out pretty quickly, so I'd say the same 5 seconds is good. If it was 20 then anybody capable of surviving, or at least avoiding the hit, suddenly turns into Samurai Jack and can "jump good" with a near 0 mass factor. :P