How long should a sermon be?

A churchman once asked Spurgeon how long it would take to prepare a two hour sermon. He responded, “three hours.” So, how long for a one hour sermon, Spurgeon said, “two days”  and a twenty minute sermon? Three days, but if a three hour sermon, I can do that right now. The art of saying the most in the least time is a sign of a knowledgeable minister.