The run time depends solely on the amount and size (capacity) of your batteries.
Total the amps on your batteries and divide by 12. Take that number and set it aside, we'll get back to it in a moment.
Total the amps on the devices you will use on the inverter, add another 1/2 amp for the inverter itself.
Take THAT total and divide into the first number you came up with.
The result will be your runtime in hours. The more batteries you put in parallel the longer runtime you can expect.