This was posted by MacroDroid which is pretty helpful if you run into problems.
There's a video codec player on the market called seamanplayer It will run a movie full length with no problem. Typical with the CM GB and Froyo builds, movies have a play time of 78 minutes max. If you're using the 2.1 Roms (sense), it'll play full length in the stock player. Just something we have to work around with to have something more on our little devices!
Also make sure they are in mp4 format..