Rest till you feel ready to go again, listen to your body. I never sit there looking at a timer but obviously the lower the rep range you’re working in, the longer the rest times are between sets and it’s usually the case that when you‘re good to go it jives roughly with the rest times you see proscribed (e.g. 1-2 mins for >12 rep work, 4-5 mins for 5’s etc).
If you’ve not recovered sufficiently from the last set going into the next one that set will suffer, and cumulatively you end up slowing your own progress down over time because you could have done more work if you’d waited a bit longer. Caveats =
-obviously if you take eons to recover from sets then maybe you should look at improving your conditioning, but I’d tackle that separately.
-proximity from failure; if you blow your load on the first set going to an RPE 9.5-10 then you’ll need more rest if you don’t want the next set to be a washout. If you’re doing multiple sets of an exercise you’ll often see it advised to either start off an RPE 8/2-3 RIR and let the RPE creep/RIR go down up each set as you try to match reps, or work with a fixed RPE/RIR range and let the reps drop off each set.
-the stimulus to fatigue ratio of the exercise. Captain Obvious, but a hard set of squats or leg press regardless of rep range requires more recovery time than a set of lateral raises or bicep curls whatever the rep range.
