Well, it’s not a rule. Not even really a guideline. More a suggestion from writer Mosse Rodbay:
I’m personally using the simple 60-60-30 rule which means I work in two highly focused 60 minute blocks of time without any interruptions where I simply turn them all off. Then I relax for 30 minutes and check e-mail and social media which I must admit is highly addictive. I hope this post will help you as much as it has helped me getting more work done while also having more fun!
Read the full piece.