I always preach 2 weeks on, 2 weeks off because there are great theories supporting that method.
But I'll throw a little wrench into it: The couple times that I ran clen straight through for 8 week periods, I noticed my best results in terms of fat loss. Cant say that the clen was the number one factor, diet and cardio and many other things surely played a part but just looking back at all the times I've taken clen, maybe its a coincidence that my best fatloss when using clen was during the times that I used it for at least 8 weeks straight, or maybe it is not coincidence. Food for thought I guess.