Six sessions a week is the number a lot of people settle on once training stops being a January project. It is a good number, and it holds up. It also rewrites the household utility math quietly, and no program anyone has ever downloaded mentions that part of it.
Run the numbers. Six sessions means six shirts, six pairs of shorts, six pairs of socks and a towel each time, plus a gi, a belt or lifting shoes depending on the discipline. For one person that is five or six extra laundry loads a week. Double it if two of you train. On top of that sit six long post-session showers, because nobody rinses for four minutes after intervals. A tank sized for a two-person household in a house built in the late nineties was not specified against that curve.
The first symptom is not a plumbing symptom, it is a recovery symptom. The shower turns lukewarm at minute eight, the cooldown gets cut short, and the mobility work that was supposed to follow it does not happen. Then the second person in the house draws a cold one and stops training in the evenings. A heater near the end of its service life fails gradually and reshapes behavior around itself long before it ever leaks. A plumbing company that services water heaters and tankless units can tell you whether yours is undersized for current demand or simply finished, and those are two different problems with two different answers.
The Las Vegas variable makes all of this worse and it is specific to here. Hard water leaves scale on the elements and across the bottom of a tank, and a scaled heater spends longer heating less water while the recovery time between showers stretches out. That is why two identical units in two identical houses do not last the same number of years in this valley. Treating the water is a separate line of work from replacing the heater, and it is the one that decides how long the next unit lasts.
If you are adding volume this fall, the honest checklist has three items on it that are not training. Enough clean kit to survive a missed laundry day. A plan for hot water when two people train at overlapping times. And a look at how old the tank actually is. The first item costs a trip to a sporting goods store. The other two are ten minutes on a Saturday with a flashlight and the label on the side of the unit.