How low will you go? More intensity means more rest.
Typically, the time frame of resting falls between 30 seconds and 10 minutes. Yeah, that's a long scale. It is affected by many factors, including your goals, the intensity and volume...The main reason for your rest is fatique and recovery. You need to rest to recover all the ATP and CP lost in the work set. On longer sets there is also the factor of anaerobic glycolysis, but it's not something that's important now.
If you are staying between 6-15 reps, and your main goal is hyperthrophy, the optimal rest is from 30sec to 2 minutes. Not longer than 2 minutes according to some theories. When going lower reps, say 3-5, the numbers are more loose and go from 3 to 7 minutes easily. Bigger rest times come to play when doing stuff like 2RM and 1RM.
Get the rest of the timeframe (30sec-2min), choose it by feel. The point in hyperthrophy is to fatique and seriously overload the muscle with work. If the set feels a bit light and not that bad, take rest for only a minute or under. If it's like hell, then take two minutes. Don't take your sets to failure, leave one on the tank. Failure training is appropriate in some points, but not on the long run. If you fail on the last sets after 2 minutes of rest, you got too much weight.
Really nice info. Thanks.