One shower runs out of water in 7min.. Other stays hot forever!

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?