Why is my hot water running out quickly regardless of other household water usage?
If you have an electric hot water heater, it may mean that one of your two elements has burned out.   Most electric hot water heaters have an uppper and a  lower element; the upper runs until the top of the tank is hot, which provides quick recovery time, then when the top thermostat is satisfied, it transfers power to the lower thermostat, which heats until the entire tank is hot.

If you are getting less than a full tank of hot water, the most likely cause is the bottom element burning out.   Less likely, either of the thermostats could be faulty, which could be keeping the lower element from turning on.

If the upper element were burned out, you most likely wouldn't get any water, since the lower element is usually not turned on until the upper one shuts off.