Electric water heater?

If you have a new-ish shower head, it's 2.5 gallons per minute. If you're using half hot/half cold, that rate will come close to using all the hot water in 20 min. An average quality electric water heater can take several hours to reheat from a cold water start.

Solutions to allow a longer shower include getting a lower flow shower head (if you have an older one with a higher flow rate), turn the water pressure down (don't turn it on so hard), take a "Navy shower" (turn the pressure down or off when soaping) and turning up the water heater temp setting (but be careful of the burn risk, especially if there are children in the home). OR, get a larger water heater.

If it is an electric water heater, it is possible one of the heating elements is burned out. There are usually 2 elements, and if one is burned out it will heat water, but very slowly. Just a guess.

Norm

I have a two-bath/washer/dishwasher home. I wouldn't have less that 50 gallons in my house.

Thanks for the info. I forgot to list that the water heater is gas not electric.