It all depends on wiring size, and the discharge rate of the batteries.
Batteries will last longer if you recharge at/before 50% SOC, which is 12.2V on static batteries.
I have a 1500W inverter hooked up to my batteries with less than 3' of 2GA cable. When I am using ~1500W the amp draw is 128 and voltage drops nearly instantly to 11.65. Online calc's tell me I am getting about a 1% voltage drop so I know I have adequate wiring. Voltage drops under use, the higher the use the more the drop.
If you want to know how many amp hours you have left then look into a monitor like the Trimetric 2020.
I normally boondock and just use my voltmeter to tell me the SOC, and now have an ammeter to know what I am using.
