It's in the rules...

Law 27: Appeals. Though some bowlers - indeed, entire teams - seem to be given to making fatuous appeals when a batsman is clearly not out, it must be remembered that the umpire cannot give a batsman out unless an appeal has been made. By the same token, a batsman is within his rights, if somewhat in breach of etiquette, to remain steadfastly rooted in his crease until someone has enquired "How's that?" even if his stumps have been sent cartwheeling into the outfield.

The batsman cannot be sure he has evaded justice, however, just because the correct appeal - and an unqualified "How's that?" is taken to cover all possibilities - is not made immediately. An appeal can be accepted and upheld at any time before the bowler begins his run-up for the next delivery, even between overs.