If you have never cruised I would do a 7 + day cruise to see how you like it.
But if you are experienced cruiser you will probably do fine.
We did 32 days this year and found we could have stayed on longer but the ship was already full up
Alexandr Pushkin, Carnival-Holiday, HAL-(old)Westerdam, Orient lines-Marco Polo, Royal Olympic-Olympic, NCL-Dream, Dawn-Princess, ACCL-Grande Caribe, Oceania-Insignia (3), Regatta(4) Marina (1) St Lawrence Cruise Lines-Canadian Empress
Scenic River cruise June 11,2012