If you want a more accurate reading on how much time is left on your laptop battery, try using BatteryBar which is a free windows application. It's a simple battery meter that gives much more accurate readings of how much time you have left on your battery.

Another cool feature is that it keeps historical data on your battery life and it then gives you graphs over time on you battery lifetime. This makes it easy to see if you are currently using more or less power that you were previously. So you can see if running certain applications draw more or less power.