Running bad and playing poorly for some extended period of time are two different things. But playing expertly all the time can result in some long losing streaks too. I don't know the exact answer but I would guess that a full time pro could maybe face a losing streak of 3 or 4 months. This would of course be rare but will probably occur at some point in ones career. If you want a better answer refer to Gambling Theory & Other Topics by Mason Malmuth. He goes into great depth on the subject in that book.