What has developed in our group is an unspoken practice of GMing for about 18 months and wrapping up with a clear climactic conclusion. No one has told me to do it as GM, but it is clear that this how the guys who have been in the group the longest do it (intentional or not, I don't know). Our Monday Night Gaming Group campaigns have all tended to look like this:
This is a descriptive diagram, not prescriptive. There is no rule, agreement or statement that says a campaign has to follow this pattern in our Monday Night Gaming Group. They just do, after about 18 months or so (40-50 sessions).