Generally each time the battery runs dead the life is cut in half and unless you determine the fix for stopping the batteries from going dead again, then another cut in battery life of what is remaining will happen, and so on and on. When the battery won't take a charge because of low voltage, I have a battery charger that has a reconditioning feature that takes about 24 hours to complete and it seems to extend the life somewhat.
I experienced this recently while over a couple days working on a project in my TV Ford E350 Quadvan that has many interior lights, power step, exterior lights, etc. that go on and eventually off every time I open a door. After a couple days of this while not having started the engine to charge the battery I found it was dead. The same thing happened again on another TV project only this time in about a day the battery went dead. The final blow to the battery was when I left a map light on over night, dead again. I learned the hard way the FIX for me was to replace the main battery and install a dual (deep charge) battery system because I can't stop the power draw opening the doors or my occasional senior moment that will happen again leaving that interior map light on.
My "how to" E350 aux battery install project is found here: