Longest 18 days: Baltic and Northern Europe.
Shortest 5 days: CM Cruise of the Century.
Though the Century cruise was wonderful and I had a great chance to meet many Cruisemates it cemented the fact that it isn't worth it to cruise for less than seven days.
We have done pre and post cruise stays in the ports for as much as a week. We went into Hong Kong five days before our cruise and REALLY loved it.