In a tank heater that suddenly got worse, it is usually a broken dip tube or a failed lower heating element — both repairable, and far cheaper than a new heater. If it has always been marginal, the tank is simply too small for the household.
Did it change, or has it always been like this?
This one question splits the diagnosis, and people skip it. A heater that used to be fine and now gives you eight minutes has a fault. A heater that has never given you a long shower is undersized, and no repair will fix that.
If it changed suddenly
The dip tube
Cold water entering the tank is supposed to be carried to the bottom by a plastic dip tube, so it heats from underneath and the hot water at the top stays hot. When that tube cracks or breaks off, incoming cold dumps straight into the top of the tank and mixes with the hot you were about to use. The symptom is exactly this: a shower that starts hot and goes lukewarm fast. It is a cheap part.
The lower element, on an electric heater
Electric tanks have two elements. When the lower one fails you are left heating only the top third of the tank, which gives you a short burst of properly hot water and then nothing. Classic and very common. An element is inexpensive; the labour is draining the tank.
Sediment
Hard water leaves mineral scale in the bottom of the tank, which insulates the burner or element from the water and steals capacity. If you have never flushed the tank and you are on hard water — which in much of this area you are — that alone can cost you a third of the useful volume. Flushing annually is maintenance most people never do.
If it has always been marginal
Then it is sizing. As a rough guide a 40-gallon tank supports one to two people, 50 gallons two to three, and beyond that you want 60 or 80, or a tankless unit. Back-to-back showers in the morning are what expose an undersized tank, and turning the thermostat up to compensate is a burn risk rather than a solution — anything above about 120°F starts being genuinely dangerous for children and older adults.
The one that is not the heater at all
If the shower specifically goes cold but the kitchen tap still runs hot, suspect the shower mixing valve or its anti-scald cartridge rather than the tank. A worn cartridge bleeds cold into the mix, and it presents exactly like running out of hot water. That is a cheap part in an inexpensive valve and a specialist part in an expensive one, but either way it is not a new water heater.
When to replace rather than repair
Tank heaters last roughly eight to twelve years. Past ten, with rusty water, a weeping tank, or repeated element failures, money spent on repairs is money spent twice. A tank that is actively leaking from the body is not a repair at all — it is a flood waiting for a weekend.