For most of the last few days I have been tinkering with the M400 for short periods and then charging it up again - without running it right down.
As a last hurrah before handing the demo unit back tomorrow I decided to get an idea of the battery life. Not having much time left with the device I decided to go for the worst case. I cranked the brightness up to full, turned wireless on and started browsing away. This got me a bit over 3 hours continuous use. So at worst with the M400 you can expect 3 hours running time. Turning down the brightness and tweaking the power settings using the Toshiba Power Management tool I would expect that you could stretch this to over 4(have not tried it). This the extended battery I suspect (totally untested) you could get a full day.
Here's the power save settings I used for the test...
