Mate in 50.
Mate in ___ - longest mate wins!
obviously you have to send the mate in 50, you can't just claim you have one...
and vanillaknight would u please stop posting obnoxious comments on all my topics

For a hundred you'd have to sac a rook on move 50 and then arrange it so you can mate your opponent on move 100 making sure not to repeat the position.
So I'm saying 100 is the longest mate, but I'm too lazy to play it out. Maybe later.

You could probably look up the longest known mate in an endgame tablebass. But the longest possible forced mate is probably an open question (and likely will be for a very long time!)

There's a thread here already that shows a mating problem in 290 moves.
Rather than repeat it, here's a link:
http://www.chess.com/forum/view/fun-with-chess/longest-checkmate-problem-in-history?page=1
Who can find the mate in the most moves?
Note: Don't post unless you're setting a record! If you want to post your puzzle please post it before someone breaks that record.
To start off here is a mate in 1. Hopefully someone will come up with a longer one than this!