I've had an adjustable standing desk for several months now. I stand at it all day, unless I have a meeting, then I sit in the meeting room. Recently, I learned that you are only "supposed" to stand for 30 mins at a time. But my chair is too low for my desk, even with the desk lowered all the way down, I still have to reach up to the keyboard and can't relax my arms. So it's very uncomfortable for me to sit at my desk. Since sitting is so uncomfortable, is it ok for me to stand all day? What consequences could come from that - could the desk get damaged from being in the "up" position all the time? Should I put it down at night before leaving?
I find it unusual that you're asking whether you should only stand at 30 minutes at a time "for the sake of the desk". The desk will be fine. Standing desks are typically designed to be able to adjust their height via buttons/switches on their side and can quite happily remain at whatever height you choose for as long as you wish.
As you mentioned in the comments that you're worried about your usage pattern of the desk will cause problem to the longevity of the desk, I'd say reach out to the admin / facility team in your office and check if they have any specific usage guidelines.
If they don't have one, or the guideline does not mention any time period for the use, then you don't need to be bothered.
On a separate note, I'd say, pay more attention to your health rather than that of the desk. Take regular breaks (in 1-2 hours interval) while using standing desk (to walk around, stretch your legs) and ask (or demand) for a better seating arrangement.
Talk to your boss and request a new desk and/or chair that will allow you to sit comfortably as well as stand ( if you want to use it as a standing desk ). Standing all day long does not seem very comfortable or healthy, and sitting with a desk that is too high is also uncomfortable and probably unhealthy.
An alternative would be to ask your boss to relocate you to a new desk/chair that is more comfortable. Make sure that you let him know that your current set up is affecting your health and ultimately your productivity.