Well, for my last show I used it for 6 weeks straight... I nearly killed myself, literally, I lost about 50lbs of muscle and not much fat (my diet was wrecked - I was eating mainly watermelon trying to keep alive). I just scrwed it.
In the past, when I used it properly, I would use it 4 days a week, for as long necessary to lose the fat. But keep in mind, this was before most people knew abou tit (circa 1996 - 97) so I didn't know about the 7 days straight stuff.