Running on the water pump, I turned my shower hot and cold full on and caught the water in a 2 gallon bucket, it took 1.5 minutes to run 2 gallon. Length of time for shower to start running cool depends on the temperature of your cold water you are mixing with your hot water. On my MH with a 10 gallon water heater, on average, we could easily go 10 to 15 minutes without running out of warm water. 15 minutes would be 20 gallons of water. We had a 100 gallon water tank so unless we were off the grid for several days we never worried about it. Normally we were in an RV park every couple of days to restock.
I would think under the same conditions with a 6 gallon heater one could go 8 to 10 minutes. Who needs a longer shower that that?