How do you get rid of guests?

Two weeks ago we had some friends of my girlfriend (who is beautiful and also real) over at our appartment. Our initial plan was to eat lunch together, maybe talk some more minutes and then everybody goes their way. We did not want to just throw them out, so most of them managed to stay there to watch a movie and then sit around for 3 more hours.

Don't get me wrong - I don't have a problem with them staying for a few hours, but if it takes the whole day, we will just not invite them over anymore.

Any tips on how to limit the duration of their stay?