I would say for about 3-4 weeks on, as it sometimes takes around 2 weeks to get a real good Thermogenic effect going, and I believe that around 3-4 weeks into the cycle, you start to lose the effectiveness (the rush) of the Ephederine, but the Thermogenic effects will still be noticable.
So in actuallity if you do a 4 week cycle, you are going 4 weeks on, then a week tapering off (to diminish any adverse side effects from withdrawal), then however else long you want to take off