I moved to a stand up desk as well a few months ago. I found the first week or so a bit tiring on the knees and feet and found myself often over extending my lower back as I stood. I am still getting used to it though and I found it makes great improvements for my upper-cross syndrome .
Standing like a statue for hours on end isn’t very comfortable, so taking a walk around the office every half an hour seemed to take the stiffness away. I am trying to ‘fidget’ more while I stand too which makes it easier to stand for almost the entire day.
I dont have a time ratio of sitting vs. standing, but I basically reserve my sitting to meetings, commuting, lunch and the occasional 15 minute sit break to give my legs a rest.