hi Bob, In reply to your question, usually the time spent between sets is just long enough for your breathing to stabilise before your next set, but, there are many differant ways to weight train, for example if you want to improve your endurance then maybe you should just go for failure that is get as many reps out of a slightly lower weight than you may be using, also you could try working the opposing muscle in between sets for example if you are working your quads (upper thigh) then while you've got the blood pumping in that body area maybe perform a set of leg curls or deadlifts for the hamstrings hope this may be of help to you and good luck from traceyh