Take a Break is a very useful addon for Chrome. There are lots of different time frames that you can choose from. When you start the addon, you will be prompted with the options page. There is an ON|OFF switch that lets you enable or disable the addon.

When you click on the ON switch, the addon will start working. When you are using it, the default notification type is opening a new tab. If you want to change it to a desktop notification, please visit the options page and mark the related option. You will be able to see your break notifications in the notification center.