There are many variables that may change the length of time a water heater will heat a tank of water. The age of the heater, the type of heater, volume of liquid and even the temperature of your house can all have a bearing on length of time. It is also important to remember that you can now set heaters to different temperatures, which will also vary the boiling time. An average sized house will need a 50 gallon electric water tank. Assuming the water heater is quite new, made and installed within the last five years; the tank will take half an hour to heat during the summer. In winter it will take a little longer as the water is probably colder. In this circumstance you should expect the tank to heat in 40 minutes. If your water heating system is gas powered then you will find that the average volume the tank can hold will be smaller, roughly 40 gallons. But gas will take a little longer to heat water; hence a smaller gas tank will take similar times to heat. If your water heater is older larger, or energy efficient then the length of time taken to heat a full tank could be considerably more. In this instance it could take an hour or more to heat a whole tank of water. It is possible to buy water heaters that heat water instantly. This may be an option if large quantities of hot water are needed on a regular basis. Heaters such as this work by heating the water in the pipe as it travels to your tap and have the added benefit of taking up much less space in a house.
