I live in an apartment complex where the rooms/bathrooms are identical, and there is one gas water heater for each unit. My shower will stay hot as long as I want it to, but my roommate has to rush through her showers just to keep it lukewarm. We don't shower at the same time or run anything else that uses the water at the same time. Can anyone help us figure out why this is a problem with only one shower?
So this is happening in separate bathrooms that is supplied by one water heater? You'd be surprised how quick you can go through 38 gallons of hot water especially with a high flow shower head. Newer shower heads are usually rated at 2.5 GPM, while older ones were much higher. Also check the timer on the heater. One other question, is the hot and cold water tied into one shower valve? Or is it two separate valves for hot and cold? If it's only one valve that ties the hot and cold together, that may be the problem. They use rubber parts that get swollen and wear over time and the valve may be the problem. That would explain lukewarm showers due to the 50/50 cold and hot mix.
Are you saying that your apartment has one water heater that only you and your roommate share, or that your complex provides hot water to all units?
If the complex provides hot water and if both showers get cold together at about the the same time, your plumbing is probably normal (your shower doesn't have some extra source of hot water).
The difference most likely is that you two don't typically shower at the same time of day; you shower at a time when less other people in the complex are also showering.