Toshiba claims that the T135's included 6 cell battery lasts for 9 hours and 22 minutes using the MobileMark 2007 benchmark. Here at HotHardware, we've been using BatteryEater Pro's Real World Scenario to test battery life in various machines, which is much more strenuous than MM07. Under this benchmark, the T135's included 6 cell battery lasted for 4 hours and 48 minutes. Obviously, there's a discrepancy here, but even a nearly 5-hour battery life is very respectable.
In looking at the graph above, you can see that the Toshiba T135 held up well compared to many other notebooks and was only beat by a few netbooks. In addition, one must consider that the display brightness was nearly maxed and Wi-Fi was on during the test, and both of these will drain the battery more quickly than if the screen was dimmed and Wi-Fi was disabled. Bottom line: even 288 minutes of runtime is a very good score.