As others have mentioned, I doubt you will need the iPod for the entire trip. If you are traveling at night, you will probably sleep for half the flight. If you are like me, and like to fall asleep to music, use the 'Sleep' function on the iPod so it turns itself off after a while.
Here's my typical flight:
1) Board plane, find seat, stow carry on stuff (don't waste iPod battery while waiting for doors to close, when they will ask you to turn it off).
2) Push off, taxi, take off, reach 10,000 ft - 30 minutes you can't use iPod.
3) Flight attendants come by and ask if you need something to drink - I usually wait for this because if I try to listen to music before this I might fall asleep and I hate being awakened. - That's another 15-30 minutes depending on size of plane. Go ahead and order something to drink. You would be amazed how dehydrated you can get on a plane ride.
4) If the flight attendant is cute, flirt profusely - Another 10-15 minutes until she shoots me down

5) If there's a cute passenger next to me, flirt profusely and/or make conversation - Another 30 minutes to 1 hour before one or both of us starts to get sleepy.
6) Break out the iPod, pick out a long playlist and settle back. I usually fall asleep sometime during the playlist.
7) About 30 minutes before landing, you'll be asked to put away all electronics.
That adds up to 1 1/2 to 2 hours, not counting any time spent sleeping or talking with your friends. Your fully charged iPod is likely to be all you need.
