It is a strong function of how many people are there.
(based on the experience of my wife's and my ceremonies, both in Dallas area convention centers):
The ceremony is short (probably less than a half hour). But, they spend a long time getting everyone seated, and then they wait quite a while for things to actually start. Afterwards, then need to hand out the natz certs. For both ceremonies, we left around 3 hours after the listed (the the applicants) start time.