Your batteries SHOULD never be discharged greater than about 50% which would measure about 12.2 volts (for a 12 volt battery). The more often you deeply discharge your battery the quicker your battery will die. Battery life is measured in charge/discharge cycles and deep discharges are worst of all. They can cause plates to warp causing the battery to short out internally.
Great information above on how long it will take to drain them when in use but I'm talking about how long they will remain usable at all. When dry camping it is BEST to run the generator daily and try to keep the battery fully charged as much as possible.
