How many characters from "Lost" does it take to screw in a lightbulb?
(At this point, when the person you're telling the joke to is in the middle of saying "I don't know, how many?" knock him unconscious with a baseball bat, put a hood over his head, drag him into a cage in the middle of a room with a two-way mirror, stereo, and intercom, turn off the lights, and leave. Come back two weeks later with a tape recorder, dice, a gun, a plate of eggs benedict with home fries, an REO Speedwagon cassette, and an old bus transfer slip).
Repeat until it "gets old" (i.e., indefinitely).