I've used a similar app that made a beep after a certain number of minutes, but I made a habit of ignoring it after a week. Now I use Breaktime (only for OSX),

I've used a similar app that made a beep after a certain number of minutes, but I made a habit of ignoring it after a week. Now I use Breaktime (only for OSX), which inserts itself in the system tray part of the screen and displays a big overlay when "time's up". It is more effective because it forces me to get up. You could build a browser notification that gets the focus of the user.