WorkWeek is a convenient app for tracking the amount of time you spend at work (or other location of interest). Using your location WorkWeek logs when you arrive and leave work. Using these times you can set a reminded to notify you at the end of your work week.
Just leave the default notification set to 40 hours and you are done. No more watching the clock. WorkWeek will notify you when your week is over.
Continued use of GPS running in the background can dramatically decrease battery life.