How long should I rest between sets? Depending on your goals, rest times may vary. On compound exercises that use your whole body (squats, deadlifts, bench press etc) between 90-120 seconds is ideal. On isolation exercises (bicep curls, cable exercises, leg extensions etc) 45-50 seconds is ideal.
The exact second is not the important aspect, however. The most important thing is that you rest enough that you are able to complete the reps and sets allotted. Do not rest too long that your heart rate decreases low enough for injury.