3500 miles with regular oil, or about 5 with syntethic. I personally do it when the oil is dark, as once it reaches that point, it begins leaving deposits inside the engine.
yeah I probably should stop this addiction. I think it might be why the oil is dirty at 2500. But then again it never was dirty with a fram before I reached 100k milesI use Castrol 5w30 oil and a high quality filter, not Fram.