A friend of ours from Puerto Rico said that in her culture setting an end time would be considered very odd and even rude - like kicking your guests out before they even get to the party. Someone else (a New Englander, if it makes a difference) made the argument that how would guests know if they're overstaying their welcome if no ending time is specified.
For our part, we don't usually set an ending time, but we've also never really had an issue. If the party is going strong, generally we want people to stay no matter the hour of the night. And if the party is obviously winding down, guests don't usually linger. Also, our crowd of friends generally keep the same hours, so everyone's idea of an appropriate ending time tends to be the same.
What do you do?