This calculator will help you determine the working time of your battery bank. You will need to input the total battery bank capacity in Ah and the total voltage V. You will also have to choose the battery type to determine the DOD%. Of course, you could also enter this value manually.
Finally, you must provide your inverter power in kW or W and its efficiency. This will give the duration when working at the maximum load of the inverter. You could input less value if you are working most of the time at a lower power. The result will be automatically calculated in hours and minutes.
For more details on using the calculator and to know how exactly battery working time is calculated, you should read this article: How Long Will a Battery Last With an Inverter?