Well, I think what we are trying to get across here, is that you need to allow adequate rest between sets for your muscles to rejuvinate themselves just long enough so you can endure your next intense set. Depending on how big the muscle group is you are working (like what Cursor said), and how much intensity you are putting into a particular set, will dictate how long you need to rest that muscle.
You certainly don't want to wait 20 minutes, or even 10 minutes between sets because your muscles will start to go cold, your
body temperature drops, your heart rate drops etc....and you don't want to let this happen. You want to continue working that muscle while it is warm and the blood is flowing to it! Allowing your muscles to rest just long enough so that you can apply
high intensity to your next set, should be a good guideline to follow...and for most, this is a minute to a couple minutes for smaller body parts worked with high intensity, and UP TO 5 mins between sets for larger bodyparts worked with high intensity.
I hope this helps ya!
-Todd