Undercroft or The Quest would keep you busy for an entire flight (and much longer! ) If you prefer Diablo type games, there's Underworlds or Dungeon Hunter...
Battle for Wesnoth HD is only $.99, and lasts over 300 hours, and is a TBS, which is pretty close to an RPG. Flight Control HD might last a bit of time, also get Dizzypad HD and Harbor Master HD which are both free, if you like either, you can get in-app purchases with more content. Warpgate HD $7.99 and has pretty good longevity. EDIT: LOL, Warpgate HD is $7.99, not free. I don't know what I was writing.
FREE with dlc: Compression. The base mode is really fun and there is dlc avaible with in app purchase but if course not without an Internet acess.
Eliss! The game is perfect for the iPad- a completely innovative touch-screen experience. It's a really crazy, sort of puzzle, sort of action game with neat graphics and a nice soundtrack. Plus, it doesn't take away much battery for the long flight, even though it has a lot of good content.