How long should a person lets muscles rest before working them again?
 

48 hours?

It depends on if you're a beginner lifter or advanced lifter (or somewhere in between). A beginner can get away with doing a full body workout 3x a week. I find that when I started lifting heavier weight, I could only do one heavy day on my upper body and one heavy day on my lower body per week before getting burned out.

You more or less just have to listen to your body and know when to stop before overtraining.

IW