I'm not an expert here but this is my thought and we have no problem with hot water lasting.
How long hot water will last has a lot to do with the valve. Stating the obvious here, the more the valve is open the quicker the tank empties.
Here's something you may not realize or given any thought to, when the hot water exits the tank this is being replaced equally with cold water. The faster the hot water exits, the faster the cold water enters.
The hot water that is in the tank helps
to warm the cold water. This will assist the burners that usually kick in fairly quick. It takes a lot longer to heat 6 gallons of cold water then it does to heat 6 gallons of warm or slightly hot water. What I'm getting at here is to ease up on the hot water.
What my wife and I do is turn on the hot water low and use the cold water to adjust the flow and temperature. If it's too cold turn the cold down not the hot up. You have to find that happy median where you have sufficient/desired flow without draining the hot water from the tank in 2 minutes. Using that pause button would help you out also.
We have a 6 gallon, with an 18 gph recovery tank. Many times my wife and I have taken showers one after the other (showers are small aren't they
) and never have we run out of hot water, well not that I can remember, and yes flow/volume is decent, not like home but then again not like a pail overhead with pin holes either. Oh and guessing here I would say each shower maybe around 10 minutes.
Oh ya, we also have one of those Oxygenics.
Hope this idea helps, does us