Every twenty minutes
Look at something at least twenty feet away
For at least twenty seconds
We often engage ourselves staring at the computer screen so much that sometimes we even forget to blink our eyes. Keeping this in view, ophthalmologists encourage us to take an eye-rest at regular intervals. They recommend a 20-20-20 rule, as stated above, to avoid eye strain.
Keep this web-app + your sound open.
It will remind you every 20 minutes to take an eye-rest.