I never did much with the custom Fedora+Sugar OS that comes on the XO, so I can't comment on it's battery life. I doubt it's going to make much difference either way, though. Under Ubuntu with the backlight turned off, I can read for several hours without power being a problem. I haven't timed it down to going dead and don't usually read for more than three or four hours at a sitting, so I don't know exactly how long it will last. You may want to ask about battery-life in the OLPC News Forum thread for Ubuntu, as I'm sure somebody has tested it:
http://www.olpcnews.com/forum/index.php?topic=4053.0
If you want some extra juice while away from AC power and are willing to carry a little more weight, the Tekkeon MP3450 external battery (
http://www.amazon.com/Tekkeon-MP3450.../dp/B000S9JWJI) comes with a tip that fits the XO and should about triple the battery life. I carry one when travelling.