Vince Gironda advocated resting as little as 10-15 seconds between sets, and even recommended working to such short rest periods that the hands never leave the weight.
However, that doesn’t mean that everyone should do this right right from the start. As Vince, and now Ron Kosloff, stressed, these are goals to be worked towards.
As Ron keeps reminding me, if you put a guy who has never trained on an 8×8 program with 10-15 seconds of rest between sets, you might give him a heart attack. Instead, you have to work toward this goal.
Perhaps at the beginning 30-40 seconds will be necessary, and then worked down from there. As Vince stated in “Developing Muscle,” “…shortening the rest interval between sets is a form of Progressive Resistance.”