Basically the short of the long of it is that you should not sit or stand for longer than an hour without doing a few minutes of the opposite.

Otherwise you will start experiencing accelerating fatigue.

Places of work that require standing should prioritize letting workers sit for a couple of minutes once an hour at minimum. It retains 95% of productivity (if not boosts it when fatigue goes down), and reduces fatigue by at least half when compared to perpetual standing.