I'm usually in the gym for weight training sessions 3 times a week. I always do 10 minutes on the elliptical for warm up and then 20 - 30 minutes of cardio after my 30 minute weight session. I try to keep my heart rate at 125 - 140 during the cardio after training.
Seems to work for me but then I'm 59 years old and have two main goals: keep the muscle mass and get rid of the fat!