It all depends on how hard you treat it. I have been putting 500/500 through mine for the last 2 years, but I rarely slam the gears, and it has never seen slicks. Ford rates them at 300ft-lbs, so anything over that will reduce the normal duty life.
many factors, as Shaun said. one would like to keep it from being the weak link in the driveline - a sticky clutch (which should be the weak link) and slicks and hammering gears can make the trans the weak link real quick.
FWIW, trannies (and some clutches) are usually rated by torque, not HP.