The C-Time App for iPhone records this automatically for you!
C-Time for iPhone records automatically for you, how much time you spend at certain places (work, school, gym, ...).
It is really cool and records all this automatically without user interaction. C-Time registers automatically when you arrive at a place (region) and when you leave it, and stores the times for it's book keeping.
Check it out!
You may get it from:
http://itunes.apple.com/us/app/C-Time/id553940406?mt=8