It depends on your goal: the American College of Sports Medicine (ACSM) recommends 3 to 5 minutes of rest between sets for heavy strength and power work, 1 to 2 minutes for muscle-building (hypertrophy) training, and under 90 seconds for muscular endurance training.
The longer breaks matter when you lift heavy loads, which the ACSM defines as roughly 1 to 6 repetition maximum (RM), the most weight you can lift for that many reps. Those rest periods help you keep intensity high on each set. For hypertrophy, the ACSM emphasizes loads in the 6 to 12 RM range, with multiple sets and higher overall volume. Power training uses light loads, about 0 to 60% of your one-rep max for the lower body and 30 to 60% for the upper body, moved fast for three to five sets with 3 to 5 minutes between them. Endurance work flips the pattern, with light to moderate loads of 40 to 60% of one-rep max for more than 15 reps and short rests. The ACSM adds that these guidelines should be adjusted to each person’s goals, physical capacity, and training status.
Image: Fit young man doing deadlift exercise in gym, by Nenad Stojkovic, CC BY 2.0, via Wikimedia Commons.