All depends on your current BF and how much fat you're looking to cut. If you have a fair bit to loose and are planning a lengthy cut (>12 weeks) I'd say wait until you start stalling out and are at a point where dropping calories and/or adding more cardio would be counter productive. If you just have a few pounds to loose (<10 lbs.) I don't see why you couldn't start sooner. Most guys here don't recommend staying on Ephedrine too long (4-8 weeks).