You can choose whatever time you want it to shut off. I don't usually do it by voltage because I haven't really analyzed how the voltage drops and when it will die and all that. What I do is charge it fully once, run it until it dies and determine how long it runs. Then I charge it up again, run it down until about 10 minutes before it shut off last time, and turn the pot until the LED shines red. That way it will always turn red at that point. But it may not be completely consistent.
The GC will run however long you want it to depending on what capacity your batteries have. Personally I know that 5600 mAh at 7.4 with regulators powering the CPU and GPU with generic components lasts anywhere from 2 hours and 15 minutes to 2 hours and 45 minutes. If you measure the amp draw of your whole system running together, you can calculate exactly what it will take to run a certain amount of time.
