The Take A Break application was designed to be a small tool that sits in the background and counts down how many minutes are there until you should next have a break.
Set the interval with the slider (from 5 minutes to one hour) and carry on working. When the interval is up, a dialog will open and tell you to have a break. There are plenty of resources out there to tell you how long a break should be, and how often.
Set the slider to your desired interval. The program will show you the current time (by the system clock), and what time you should have your break. It will also countdown in the title bar so you can get the window out of the way. After you click OK on the message telling you to take a break, the countdown restarts. Or click Quit if you've had enough.