About half an hour to an hour, it will take about that long.
The following is a method for heating bath water: (for reference only)
Connect the extension cord to the outlet.
Put the water heater in a bucket filled with water, insert the water heater plug into the power strip and start boiling the water.
Open the countdown program on your phone and set a water boiling time, such as the 15 minute countdown shown below. Click the start icon to start the countdown. When the time is up, the countdown is displayed. The phone will prompt you accordingly. You can unplug the power strip plug from the power outlet and unplug the quickly heating plug from the power strip. After putting them all away, you can start taking a shower.