When I was in grad school, I got in the habit of taking a break every 45-75 minutes. That's about the length of one CD, so I had to get up to swap in a new one.
Now all my music is in iTunes, but I still find it helpful to make a clear change in the playlist every hour or so as a reminder to get up and move around.
I also have an ergonomic reminder on my computer set to take a break every hour. If I'm doing heavy reading or other focused but somewhat tedious work, I switch to a different type of task at the break. If I'm writing or editing, I take a somewhat longer break, but usually stay on the same task for up to 3-4 hours.
Katherine