Whenever I'm editing or so, I'm usually an arm's length away from my screen.
(50 cm to be exact ;D)
I've got a mac, so the screen is rather flat, making it possible for me to have it as far away as possible, when I had the thick thing (the one that looked like a TV I sat pretty close to it, as I leaned my elbow on the desk, haha!
Otherwise I just sit with my feet up at my desk so, maybe 80 cm. I shift by far a lot, I'm a restless person.