Read the manual. That will tell you the expected battery life. Then again, manufacturers hype their products, so record the cat/dog/goldfish for however long it takes for the battery to go flat. That way you'll know how long the battery will last. Just remember to charge it before hand and afterwards.
Make the most of the loan and do a nice 'POV' movie with the good lady.
Seriously though I'd be interested in hearing how you get on with it, I thought of getting one but feel the shape of the Go Pro isn't ideal as a helmet camera on the bike.
My first gen unit will record for a bit over 2.5 hrs on one charge using a 16Gb card. I bought a second battery and card to extend that to between 5 and 5.5 hours. Best to switch off any wifi/bluetooth etc with the latest ones to get the best battery life. If you stop and start the unit a lot, it will decrease battery a bit, best to start it and leave it to run. I also switched off the red flashing light (every little helps eh?)
I have mounted mine under the bars since it seemed to interfere with GPS reception on the Garmin when I mounted it above, if you do this, set it to "upside down" mode so the footage comes out the right way up and you don't have to flip it in editing.
The wide angle setting works well on a bike too, you'll get this kind of thing:http://www.youtube.com/watch?v=6JCnckoF8QA
Ok, so I've had a bit of a play - figured out how to mount under the bars, and flip to upside down. Seems to work well.
However, even on good road surfaces there's a lot of crackle in the audio.
Here's a clip illustrating the effect: http://youtu.be/PrTAaK_W_G8
I've found a link from someone who packs the housing with cotton wool, I'll try that later, but has anyone solved this more elegantly?