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While this is not exact, it is typically based on heating 1 full tank from 45F to 120F. Most 40 and 50 gallon units require 40 minutes to 1 hour. Between 30 minutes and 1 hour. Depending on amount of water used before and age of heater. This depends on age and how many gallons it is heating. If it is new & just filled or you ran out of hot water, About an hour. 3kW electric on a 50 gal tank (US) = 10,236 BtuH on 400 lb water. Since 1 BtuH = 1 lb water/1 degree F/Hr then 10,236 BtuH will heat 400 lb by about 25 deg F. in one hour. If you fill the tank completely with cold water at 40 deg F and want it at 140 deg F, then it will take about 4 Hr. Same tank on natural gas at 50,000 BtuH will take about 45 min.
The recovery time is the time the water heater takes to reheat its entire supply of water, and depends on whether the heater is a gas or electric model. A gas water heater will recover in half the time of an electric unit. A 40-gallon gas water heater can recover in about an hour.