At the office I work at we're allowed to take breaks from the computer every 30 minutes for around 10 to 15 minutes which allows us to get up stretch our legs, go to the restroom grab some coffee and even if we want too we may go outside for 10 to 15 minutes during out break.
It's best to take frequent short breaks from the computer instead of staying on the computer for hours then taking longer breaks.
As long as you're getting up from the computer to take breaks often then you should be fine.