There's been a lot of threads about this, you can search for them at your leisure... but the overview is that 10.8V is the absolute minimum you should fly that down to, which is the first warning, and when you hear that you should land it pretty much straight away. However this will stress your batteries much more than if you come back and land at something more forgiving, like 11.12 volts. Slightly shorter flights, but your batteries stand much less chance of puffing, and doing more cycles if you treat them nicely by coming back a little early.