The using time depends on the capacity of the battery. According to the formula: working voltage = battery voltage * battery capacity *0.8*0.9/rated power of loads

For example: a 12V60AH battery works with a 220V 100W device

Working time=12V*60AH*0.8*0.9/100=5.184(hours)

Note: 0.8 is the discharge rate of battery (constant), 0.9 is the inverter efficiency (constant). The real using time is also effected by the newness of your battery