Your question is a bit unclear. Do you mean the purpose of the project is to plan an event, or you are showing a project as a single event on a higher tier schedule (which I would never advise doing)? Saving Changes...
The number of hours to execute the event itself will be a function of event length, number of people involved in execution, and how much they are utilized. Some people might participate for all of it while others like a key note speaker, very little.
Large events are frightfully expensive when you add up all the people involved. How many total labor hours it will take can be fairly involved as well. Events can orchestrate many comings and goings. If it's only a couple of people, you can multiply Bellybuttons X Event Length but beyond that you need a detailed SOW to add up the total hours. Saving Changes...
