NoExcusesGym

  You can wait anytime between 30 seconds to 2 minutes.

Strength training depletes phosphagens - compounds that store energy for your muscles. It takes some time for these compounds to recover. They recover 50% within 30 seconds, 75% during 1 minute and 94% during 2 minutes.

30 seconds should be enough if you are using light weights - ones you can lift 12 or more times. The same apply for routines that involve alternating the muscles you are working on during each set.

When you workout with heavier weights ( you can lift it only 6-12 times) rest for 1-2 minutes to let your muscles fully recover - or you may not have the strength to finish.

For faster workout alternate between upper- and lower-body exercises, so one muscle group rest while another works.

(by Marcela Vanharova)