Have you ever heard of the pomodoro technique? Its a formula that you use to increase your productivity. The pomodoro technique sums up to working in cycles of 20 minutes work then 5 minutes rest. After 4 of those cycles you take a 30 minute break. If we look beyond the advertising and buzz words (like increase productivity), the pomodoro acts a schedule that helps you to not get completely burned out.

There are plenty of youtube videos that go into detail and phone/computer apps that provide a time set the 20 minutes and 5 minutes cycles.