If your hot water heater is delivering less hot water than it used to, then you should know that there are several things that can be causing this problem depending on what type of water heater you have. In an electric water tank model, a lower element could be defective. In a gas model it is most likely to be the dip tube.
In some cases there is nothing wrong at all. In the winter, incoming water temperature is much cooler, which causes the remaining heat in the tank dilute much faster. Another reason for getting reduced hot water is, may be you have changed your shower head with the model having higher flow rate.
Few things which you have to know is, a water heater can give you 2/3rd of it’s capacity as usable hot water. If your shower heads flow rate is 2.5 gallons per minute, then your shower time will be close to 15 to 20 minutes before the temperature drops. If you have the 5 gallons per minute shower head, you have 8-10 min of hot water.