If you are actually following set, repetition and rest period guidelines it should probably take quite a bit less, depending on how many exercises you can fit in. Since I never seem to be able to hit the 30 second rest period between sets, and I can't always get to the machine I want immediately after my last exercise because it's in use, I always seemed to average 10-15 minutes an exercise. I haven't been to the gym in months though. :/