Increase the intensity of the workout. For an example, instead of doing 20 squats and resting for 2-3 minutes per set, do 10 squats, rest for 30 seconds, then another set, rest for 20 seconds and then another 10. You repeat that 3 times and your heart rate would be through the roof and increasing your fat burning potential instead of having it go up and down with the long rest breaks or prolonging the workout. It also depends on the goal.
If you want to burn fat, do your workouts no more than 30 minutes. High intense and locked in focus. If you're weight training to build muscle, 45-60 minutes.