Could anyone tell me what the likely battery life of an iPad 3 might be? (Or an iPad 2 - I believe they're similar). I'm planning on a five hour European flight in a plane without an auxilary power supply, and wondered how far I'd get on internal power, presuming everything that could be turned off was and the display was at minimum useable brightness - but GPS permanently on.

Unfortunately I can't answer your question but can recommend one of THESE which I have as a back-up. There are more powerful versions as well such as THIS and I would think having one in your bag is probably money well spent if the batteries fade at the most critical time, sod's law says it's going to happen one day!