"The Imperial City is a walled citadel rather than a single palace, and it takes longer than people expect. Half a day is a reasonable minimum; a full morning lets you walk it without rushing.
Much of it is ruin. The Nguyen court left in 1945 and the compound was heavily damaged in the fighting of 1968, so some courtyards hold restored halls and others hold foundations and empty ground. Restoration has been steady for years and buildings reopen one by one, which means the site looks different from one visit to the next.
Go in the morning. The courtyards are wide, open and almost without shade, and by midday the stone holds the heat. In the rainy months the same ground floods quickly, so waterproof shoes help more than an umbrella.
The ticket also covers the royal tombs on a combined pass, which is worth buying if you plan to see them. The tombs are spread along the river south of the city and need a taxi or motorbike for the day, so most people pair the citadel with one or two tombs rather than trying to see everything."