BreakTimer for Chrome
More About BreakTimer
This extension is not intended to enforce anything on you but instead to nudge you in the right direction should you choose to look after your health.
BreakTimer will notify you with a new tab and an audio alert when it is time to take a break. You will also be reminded every 20 minutes to give your eyes a quick rest.
Note: the extension may be less useful to you if you have the icon hidden. If your icon is hidden, you can resize the address bar in Chrome by dragging it from the right-most side to show more extension icons.