Fully charge the battery to cutoff. Hook your inverter to the battery. Plug a kil a watt meter into the inverter. Then plug something like a 1500 watt space heater into the meter. Run until the space heater shuts off. That's close enough without making a career science project out of it. It's not perfect because it doesn't take into account the inverter conversion inefficiency.
You could also use a clamp amp meter on the positive battery cable to measure DC current. Then get the run time off the kil a watt meter and multiply.
Note... many lithium batteries go to sleep when a certain voltage is reached (controlled by the BMS). Use a jump pack to wake it up or it may not charge.
https://www.amazon.com/Electricity-M...2813183&sr=8-9
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