It will take about an hour to an hour and 45 minutes in 40 and 50-gallon units. Your average gas tank heater takes about 30 to 40 minutes to heat water once it enters the tank. Compact Design Of The Water Heating System Saves Up To 12 Cubic Metre Of Storage Space When Compared Water Heating Systems Tankless Water Heater Water Heating Think your water heater is taking too long to. How long does it take for a hot water heater . You may be wondering exactly how long does it take a water heater to heat up. If you are staying in extreme weather conditions heating the entire unit of 50 gallons may take up a long. This initial heat up occurs when new water from your plumbing supply is fed into the tank. A 50-gallon hot water heater with 5500-watt elements set to 120 degrees takes about 1 hour and 20 minutes to heat water coming in to the unit at 60 degrees. You can also factor in less water-intensive activities like using a dishwasher or a clothes washer if. Remove old water heat...