Draining a water heater something that you should do once a year to help remove the sediment that builds up in a tank. Draining the tank will increase the lifespan of the unit. In some cases it may take some finagling to get it do drain down.
Most of the time if a water heater is draining properly it should only take about an hour to drain down. If you are using a hose to drain your water heater it will usually take a little longer than if you are draining it right onto the floor. We have had some water heaters that just don’t see to want to drain at all. In this case we will open up the t&p valve that is located on either the side or top of the water heater. Opening the tp valve will help vent the water heater and increase the flow of water.