Originally Posted by elind
Do a test of your hot water system. Take a 5 gallon bucket into the shower and run the hot water shower in it. Will the hot water get cool before the bucket is full? Or dump the bucket to see if you can get the second bucket full of hot water. This will tell you how much usable hot water you have. While doing this check the time the hot water did last.
I think this might be a bit deceptive, reason I say that is how open is the valve? A fully open valve will get cooler water quicker than a mid/partially open valve.
Best way maybe to get a stop watch and have a real shower with the flow rate you like and time this from start to water cooling. Still won't be totally accurate but it will give you a good idea.