Anecdote here: it took me around 3-4 weeks to get fully acclimatized to my standing desk, a period during which I was often tired and sore. However, once I got used to it, standing became a normal part of my day. Because I'm already standing, I have an easier time walking away from my desk - to get a drink, go to the bathroom, confer with a colleague, go for a walk, etc. - and I've also been incorporating squats, calf raises and stretches into my daily movements. Several months after going all-standing, I feel better than I have in years. My posture is better, my back no longer aches, my knees have stopped hurting, my mood is better and my productivity is as good as, or better than, it was when I sat all day.