if I was doing 3 sets of 15 repetitions of a bench press, I would leave exactly 45 seconds between sets. If I was doing 3 sets of 12 repetitions with heavier weights, I would need 50 seconds between sets, if my sets were 10 reps I would take 55 seconds, and if I was lifting heavy weights, at 3 sets of 8 reps, I would take one minute between reps. This is a good baseline for an average athlete to work with. In time, with consistent work, rest periods can be incrementally shortened even as muscles grow and are stressed to their larger healthy limits.
