Like many things, it depends. In general, after every set you should rest long enough to:
- No longer be breathing super heavy.
- Feel mentally strong and ready for another hard set.
- Not be crampy in a supporting muscle (like being fatigued in your lower back before another squat set, for example.)
- Be recovered enough in the target muscle to be able to do at least 5 reps on your next set.
So long as you’re resting long enough to check all 4 boxes, you’re resting long enough! Because some exercises make you more tired than others, the amount of rest time may differ wildly between exercises. For a small muscle exercise like the calf raise, you might only need 10 seconds to check all the boxes before your next set. For a large muscle exercise like the squat, you might need 3 or even 5 minutes of rest between sets. You can also rest longer if you like, but no shorter than what it takes to check all 4 boxes!
The above strategies apply to normal sets. There are some instances where you may want to reduce rest intentionally, like doing a super set or myorep match set. These rest times are specific to the intensity technique you are employing and should be looked at differently.
To watch an in depth video on this matter, check out the video below: