Actually, it is pretty simple to figure how long it will last, but it is a function of flow rate, tank size, and the heating capacity. The following is a bit simplistic but realistic, realizing that the actual temperature curve goes from full hot to diluted over the first two or so minutes and I am assuming a contant mixing ratio.
Say you are taking a shower at 4 gallons per minute, with the water temperature coming out at 110 degrees - what most people consider good shower temperature: a 130 degree water heater setting, without additional heating, taking in fresh replacement water at say 50 degrees, will drop to 110 degrees after about 1/3 of the tank capacity is used up - say about 27 gallons in your case. (The old plumbers rule of thumb is figure 30% of tank capacity as usable for a high-flow use such as filling a tub or a washer - so about comparable). That equals 5 minutes of shower time if you have an old shower head, about 10-12 minutes with a low-flow shower head, without any other uses in the house. That is JUST from the stored water in the tank, before it ddrops below 110 degrees, evenn though there would still be 80 gallons of 110 degree water in the tank that you can't get at short of turning off the incoming water and draining it out the bottom. This dilution factor is one of the things that makes hot water heater inefficient providers of hot water.
Now you add the heating capacity - say about 35-40,000 BTU typically. During a 10 minute shower, the heater would heat about an additional 1.5 minutes flow at the same temperature, or about 6-7 minutes worth added onto the low-flow rate shower.
For long flow periods that capacity of the tank does not matter - the heating capacity of the tank is all that determines the amount of water you can draw. For a 35,000 BTU heater, assume 90% efficiency, that would be about 60 GPH or about 1 gallon per minute - about half the flow from a low-flow shower or a sink faucet on full. Therefore, if you wanted an endless shower capacity, you would (without any other simultaneous uses) need about a 70-80,000 BTU water heater - about the same capacity as a typical household baseboard heating furnace, and about $2-3,000 cost.
Therefore, in the case of your 1 hour test, if only one faucet was flowing, you would have had about 27 110-degree equivalent gallons stored plus about 60 gallons newly heated water or about 90 gallons available at 110 degrees - or just about 1.5 GPM, which works out just about right for one sink faucet flowing fairly well on hot setting only, so I see no reason to believe the hot water heater is not working well. I could not find an ASHI standard for testing water heaters, so this 1 hour test must just be something that inspector came up with on his own.
An 80 gallon water heater is overkill for most houses - even a 60 gallon is considered large, so I am guessing yours has a sauna or hottub that the larger unit was installed for filling.
I changed from a 40 to a 50 gallon heater when my two daughters were young and no one ever complained about not enough hot water, and now we commonly are doing a load of wash, have the dishwasher running, and are taking a shower at the same time and, while more like pleasantly warm but not steaming hot at the end of a 5-7 minute shower, has never started to run cold on us. However, we did have a rule of not doing laundry or dishes if two people were taking showers at the same time.