Hot water runs out in 5 minutes in shower?

not sure if this applies to your set up,,,but regular water heaters have what is called a "dip tube"...the top of the dip tube is just beneath where the cold water inlet pipe attaches to the top of the water heater..the dip tube carries the cold water to the bottom of the tank so you get the hot water off of the top undiluted by the cold water...if the dip tube deteriorates then you get cold water entering the top of the tank,,quickly diluting the hot water and coming out luke warm or cold within minutes...if your tank was newer i would say replace the dip tube.but if its nearing its expected life anyway,maybe you should replace tank......sometimes the cold water inlet fitting can be hard to unscrew after 12 years,,how corroded is it....it might deform or break off when you try to remove it to replace the dip tube...also the old dip tube or whats left of it is probably fallen down into the tank where it will deteriorate further and send small pieces of white plastic through your pipes that could clog the faucet strainers,etc...is that already happening?..there is no way to get the old dip tube out if it has fallen down...its only held up by the top of it being flared out a little....as i remember,,the main cause of deteriorating dip tubes was the chlorine in city water,,do you have city or well water??is it treated??..with what??...perhaps it just fell and isnt deteriorating,esp if you dont have stuff in your sink strainers......hope this helps.....dan