I use it all day (maybe 8 hours a day), but I move around a lot. Whenever I'm not typing or reading on the screen (thinking), I walk around. That helps a lot. I sometimes go sit down a bit in other rooms, but basically I can stand all day if I have to.
It took my about 3 weeks to get to that point. For the first few weeks, my feet hurt a lot. But at some point I passed a threshold and it pretty much stopped hurting altogether as long as I can walk around and shift my weight from one leg to the other a lot (I couldn't stand still all day).
I am not Mike. But I use a standing desk. It took me a few weeks to get fully adopted, but now I can stand (or move in place) for several hours, if necessary. Usually I take short breaks though.
And actually, the ability to take short breaks and walk around is something I really cherish. Removing the (small) barrier of needing to get up has really helped.
But you can not read too much into my experience report with the standing desk: I took up weightlifting at the same time I got my standing desk, and so it's hard for me to say which change was responsible for which improvement.
It took my about 3 weeks to get to that point. For the first few weeks, my feet hurt a lot. But at some point I passed a threshold and it pretty much stopped hurting altogether as long as I can walk around and shift my weight from one leg to the other a lot (I couldn't stand still all day).