http://connect.garmin.com/activity/127060342
I use a Garmin 800. The battery life is somewhere in the ballpark of 20 hours. I spent 5 hours riding the bike on Saturday, and about 2 more thrown in for lunch on the road, and some sight seeing. From what I've heard, this is the main weak point of using a phone as a bike computer, and it probably doesn't affect that many people here.