It really all depends on a lot of things. How good is the iPhone's cellular signal (from the tower)? How far away from the iPhone is the iPad? Can you describe the conditions you are in when the iPad loses the hotspot?
I have to ask all this because, so far (and knock on wood), I can't duplicate the issue. My iPad3, on iOS 6.1.2, easily connects and stays connected to my iPhone 5's hotspot.
Marilyn