[p2k]

Open the back cover, and read what it says infront of the mAh, then take the current discharge # and divide it.

battery mAh rating
--------------------------- = Hours.
current discharge mA

If your wanting your Battery Capacity the most exact way is to fully charge your phone. Turn off any power saving and low battery alarm/shutdown, and turn on battery monitor. With nothing in the background running this discharge # should be constant. Record how long it takes before it dies, then multiply it by the discharge rate and it will give you your battery capacity.