you can usually tell by using the excersizes that you have 2 sets on. just expiriment a little and see how long it takes get your strenght back. for example if u got 2 sets of bench press and you do like 250 lbs. 12 times and you rest 60 seconds and on the second set u can do only 6 reps than you need more rest time. I use 2 minutes, I could probably use a little more time, but then the workout starts getting too long!!