The time needed for the event is affected by a number of different factors.
The age of the children will affect the time needed. Younger children take longer then older children.
The size of the group affects the time. Larger groups take longer then smaller groups.
When we find the room with all the needed tables and chairs in place, that saves of time. If we don't have to gather tables and chairs from other rooms in the school we can save a lot of time.
That said, we can usually get the job done in time regardless of the above circumstances. We can simply work at a faster pace if needed. However, when there is a time limitation we appreciate knowing well in advance.