Reps
 

Bit of a noob question.

How far apart can reps be done to be benefitial

eg do you get the same bennefits from doing say

20 pushups 4 times with maybe 2minutes rest as if you waited 20minute between reps?

you want between 30-60 seconds rest between sets, max.

The time between reps depends on your goals. You want to take a bit longer if you are going for pure bulk, whereas you want to take less time if you are going for some more conditioning.
When I'm bulking I take a min or two, when I'm conditioning/maintaining I take a minute max.