[Richard]

I have found that internal tank vs shore water makes a huge diffference in how long the water lasts. The second thing that makes a huge difference is flow rate.

If it's winter and I want a long shower, I make sure I run off the tank and I throttle the flow rate back with the shower control handle.

All of the tech info above is quite useful if you want to diagnose if you have an AH problem. It's good to know how much water is actually coming out of the shower. A five gallon bucket and timer will tell you that in a few minutes. A thermometer will tell you the temp of the incoming water.