Battery runtime is an estimate, not a fixed number of hours. The same battery can last much longer with lights and phone chargers than with cooking or heating appliances.
Use energy and power together
For a rough estimate, divide usable battery energy in watt-hours by the average load in watts. If 1,000 Wh is available to the load and the load averages 100 W, the calculation gives ten hours. That example is not a specification for a Litherra product.
If the capacity you have is a nominal battery figure, allow for the manufacturer's operating limits, inverter and other conversion losses, standby use and any reserve you want to keep. Temperature, battery condition and appliances that cycle on and off can change the result.
Compare the estimate with everyday use
Record what is running and watch the system's readings over a representative period. A fridge's average demand, for example, is more useful than assuming its compressor runs continuously at the label rating.
Also check power limits separately: having enough energy for an appliance does not necessarily mean the battery and inverter can supply its operating or starting power. For planning a complete setup, start with your daily power requirements.