How often should I take a break from my computer?

Taking a break from your screen:

"The HSE (Health and Safety Executive) suggest that short, frequent breaks are better than less frequent longer breaks, so a 5-10 minute break after 50-60 minutes is better than a 20 minute break every 3 hours."

Source: http://www.ihasco.co.uk/blog/entry/189/it-health-and-safety-how-often-should-i-take-a-break-from-my-computer

Taking a break from sitting:

"To get the right balance, Alan Hedge, professor of ergonomics at Cornell University, recommends sitting 20 minutes out of every half hour at work, standing for eight minutes and moving around for at least two minutes."

Source: http://www.cnn.com/2015/08/06/health/how-to-move-more/index.html