i had 300 on mine when i was at uni for before lectures etc. That was about 600 odd Mb. But i also had some family guy, and they were maybe 50/60 Mb i think.
I ripped them from DVD, but be careful i think theres some sketchy law regarding ripping dvds (or in general - copyright maybe? i dunno.) to the ipod or something (or so i was informed a few weeks after). I actaully only use my ipod for music now though.
When I first got my ipod I ended up ripping stuff from DVD. Unfortunately it's quite a lengthy process. Also, most movies are in widescreen and look crap on the tiny screen, so TV stuff was better, cartoons like South Park, The Simpsons or Family Guy would be ideal.
Another problem was the fact that the battery only lasted for two hours in movie mode, although I think the new ones may be better.
There will probably be good films on your 13 hr flight won't there?
Be wary of the ones you can download from file-sharing websites. A friend of mine (ahem) downloaded 'I am Legend' and it was a definate 'took a video camera into a cinema' job. Complete with mans head in front of camera.