Another factor is the width of your hallways inside the buildings. The wider the hallways, the easier the flow. Remember, the tighter the hallways or smaller the hallways, the harder it is to get the flow of people from one experience to the other. So there are many factors that must be considered. You can certainly try doing 20 minutes, and if you begin to realize that it creates too much of a bottle neck, or if you grow and your facilities are handling 250 to 300 now, you may need to reconsider moving to 25 or 30 minutes.
Traffic flow is a big issue and an easy one to overlook.
Directing people in their parking choices helps. The early arrivals should park where they don't have to go back into your most crowded area between Sunday Schools if possible. Resources are much needed. I cannot tell you how many churches I talked to and each one said they wished they had some help. I think the best resources are the ones that Don Pucik and Beth Thompson and others will offer on this transcript.