Depends on your trading style - if you've been at this awhile you should be able to lose at least $500 per day average, more on a really good day.
Are you asking how much you can lose in a day trading, or asking what you should use as a limit? You can lose all of it in one day, many things can go wrong if you not careful. When markets closed cause 9-11, I knew guys who didn't know about early closings of the markets and with a handful of Long ES contracts lost in excess of $20k when markets re-opened a week later from huge gap down. Dow opened 684 lower, airline stocks re-opened $20 bucks a share lower.
Thanks for your reply.... my question was posted in response to another thread where many are discussing how they doubt the possibility of making $300-$400 per day. I am wondering how people view it from the flip side.