I think it also depends on the number of ports. I think our 12 day cruise would be a perfect time, but we are at a port each day with only on day at sea, so with so much to do, its not going to feel like enough time.
Now a 12 day cruise with 7 days at sea might get a little old. Who knows? I guess I'll just have to plan on taking a trans atlantic cruise to see.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013