A fresh battery on the 3630 should last about 4 hours with the backlit on "Low." But the serviceable life is about 12-18 months, after that the runtime will dip lower and lower to 2 hours, then 1 hour, then 30 minutes... Eventually it will hold virtually no charge and you will get a hard reset everytime you unplug from AC.
Several companies provide replacement batteries and this istall is actually very easy.
http://www.services.foxconn.com/ for one...