There's a simple calculation to figure this out and it seems to be fairly consistent. Assume 30 seconds per person.
So, if you have 200 guests, it will take 100 minutes to serve with one buffet line 50 minutes with two lines, and 25 minutes with four lines.
What surprised us is that so many caterers think an hour or less is ok. Once we introduced this "formula" to several caterers, they started adding lines and making buffets double sided, and the service has been quite efficient.
Remember, you are styling your day. Ask your caterer specific questions about how they plan to serve your guests, and make sure that it meets what you want. Good caterers will adjust based on your input.