OptiBreak par Edward Webb

Sends a notification every 20 minutes to look away at an object 20 feet away for 20 seconds. This extension can help users follow the 20-20-20 rule recommended by most optometrists.

You can also customize how often the extension will send you a notification.

https://en.wikipedia.org/wiki/Computer_vision_syndrome
https://www.healthline.com/health/eye-health/20-20-20-rule