How long does it a fish tank heater tank to heat the tank?

It all depends on how much the tank holds and the wattage of your heater I like to have 10 wattage for ever gallon so if you hVe a ten gallon tan you need a 100 watt heater also the temperature of you want will make a deference normally my 20 gallon tank can go from 46F to about 77 F in two hours