Dedicating a whole day to the Forbidden City is not excessive. The complex is immense and every courtyard hides fascinating details.
I booked the ticket in advance, which is essential. After the visit, I took advantage of the fact that the Temple of Heaven was not far to combine both in the same day using transport in Beijing.
If you are organising your plan to travel to Beijing, I recommend leaving the Forbidden City for a day with good weather: much of the route is outdoors.