I've been using a standing desk for about 2.5 years. I stand almost all the time but I tend to take few mile walks every few hours to break up coding / thinking, etc..

I no longer do it by a schedule because my mind will tell me when I should go, but to begin with I would just set an alarm and go get up every 2 or 3 hours, and if you're close to finishing something important (say 10 minutes), finish it first and then go.