With this calculator, you can calculate how long it takes for the battery to discharge with a certain load. You can enter the capacity and voltage of the battery, as well as the power or current of the device as a load. Since the battery cannot be completely discharged in real life, it is also worth using the lower limit of the capacity, which is at least 20% for lead-acid batteries, preferably more.
Please note that the results are indicative, as they are based only on calculation. In reality, the quality, age, condition and operating temperature of the battery significantly affect its charging capacity.