Six Gallons at 50/50 hot cold will last six minutes
Recovery depends on Gas, or electric, or both
Electric up to one hour, gas about half that, Both,, Faster. Close to immed in fact.. I'd have to do a whole lot of math to tell you how fast you can draw with both and never run out.
IF it's a problem.. Consider a replacement.. NOW This replacement has one issue, IT WILL NOT BURN ELECTRICITY.. there is one that may but I'll cover it later.
A "Demand" water heater.. IF on city water you don't run out of HOT till you run out of PROPANE,, if self contained you run out of both HOT and Cold at teh same time.
Now.. The other option
A small electric "pre-heater" can add heat to the water entering the water heater.. These can survive in a 30 amp RV.. But I don't recommend it
Also a HOT ROD Water Heater conversion (This is an electric heater that screws into the drain plug port) may add enough heat, but again I do not really recommend this.
The Demand heater is best for folks who run out a lot
I'd like to see someone come out with a six gallon heater followed by a Demand unit, that way you could switch on whatever you needed .
My wife woudl run out doing dishes.. I don't.