I decided to test the battery life of my IBM Thinkpad T42 tonight. The test was very simple: start the laptop with a full battery and use it until the battery is completely empty. The usage mostly consisted in browsing the Internet over the Wi-Fi connection available in my hotel room. I didn't turn any battery saving features on. In particular, screen luminosity was on maximum the whole time. So all in all, the machine was being used in a high performance configuration, rather than a power efficient one.
The result is that it took 3 hours for the battery to go down to 10% charge, at which point the laptop beeps and the battery indicator in the task bar goes amber. It then took another 10 minutes for the indicator in the task bar to reach 5% charge. At this point, the laptop beeps again and the battery LED starts flashing. Finally, it took 10 more minutes for the laptop to decide it could not function properly anymore, at which point it automatically hibernated.
I was pleasantly surprised by the results of this experiment. 3 hours and 20 minutes of battery life for a high performance laptop is good. But the really nice touch is that it hibernates automatically rather than completely run out of power. This ensures that you can't lose data accidentally when running on battery.