I never timed mine while watching a movie, the more data the stream has the more work the CPU has to do. so a blu ray movie quality will not last as long as a compressed 640*480 one.
we use playbooks on car trips and my kids have not killed them in 7 hours.
There are some USB battery packs you can get to charge it up if need be.
Trent Battery Pack and Charger looks like it will work well. http://www.newtrent.com/Best-iPad-Ba...-p/imp1000.htm