Not entirely from experience (I did stand for hours as a cook but I had a break and was definitely moving around)
Your soles get to become sore and you tend to shift from foot to foot instead of standing on both feet at once. your hips get sore from this. If you are standing stark rigid, there will be wavering unless you flex your muscles (but that will greatly reduce your stamina in the situation.) your stabilizing muscles will get tight from firing so often to correct your posture and may become twitchy or tight. Some may not relax and would require physical massage to loosen. Do this too much and it will become too painful to massage it and it will become difficult to do simple things like walk or run. Your spine and joints will compress, decreasing height slightly but more importantly it reduces your endurance to high intensity activity like lifting or running before your joints will hurt and you may sustain injuries earlier. Your posture will suffer unless you have great mental discipline to pay attention to it. Assuming motionless standing, dehydration is a problem there should be hourly intake of some water to be normally hydrated. missing a few hours is ok but 7 hours a day with no hydration is dangerous.
TL;DR: stabilizing muscles will hurt, stamina decreased (maybe) and injury chance increased. Also dehyration.
Disclaimer: this is conjecture, I do not have any medical accreditation, nor have I routinely stood to attention for hours a day. This is a conclusion made by extrapolating my experience of standing for a long time to standing still for a long time.