The broker can technically call the shares back at any time. Available shares (for short selling) changes every day. So if you're short, and in the red, the broker can sell you out against your will if they need the shares back. You might get a phone call telling you you have until EOD to cover your position. If you don't cover, they will cover it for you.

More established companies usually have more shares available. But if you short a little company with a supernova chart, you can lose your ass if it keeps going up and the broker calls your shares the following day.