It depends. Work days, about 10 minutes and since I am balding and the hair I have left is 1/4" long - that includes shaving in the shower (best thing in the world, I never get razor burn)
In winter, usually as long as possible. Especially now that the kids are on their own and the wife and I have all the hot water to ourselves.
In the summer, I still like hot showers but towards the end, I turn off the hot and turn the cold up full.