How long should a cat be AWOL for before you start getting properly worried? My cat's been gone since Saturday afternoon, and before that he'd never been out all night long. He's still pretty young, only about 9-10 months.

Obviously I'm really worried about what might have happened to him, but a part of me still thinks (hopes) he'll just wander back in soon enough and start eating his food as though nothing's happened. So how long should we leave it before doing the whole MISSING CAT poster thing etc?

:-(