Ya..I agree with SY Chen..heater on for 5-10mins more than enough liao cos my current parents' house using one which is around 23L Rheem (not sure how to spell). Usually what I do is switch it on for around 10 mins then turn the tap to hot water and let it run for around 20 sec into a pail. Then I adjust the temp while running the water into the same pail. The hot water can be used for at least 2 persons to bathe.