Longest cruise was:

NCL Star, 12012, Panama Canal WB MIA-LAX 10/20-11/3/2014

14 day full canal transit.
Norwegian Star Suite 12012 deck 12 Starboard forward category SG
West Bound origin Miami
at Sea
at Sea
Cartagena Columbia
full Panama Canal transit Atlantic to Pacific
at Sea
Puntarenas Costa Rico
at Sea
Puerto Chiapas Mexico
Huatulco Mexico
at Sea
Puerto Vallarta
Cabo San Lucas
at Sea
Long Beach (San Pedro port of LA)

That is the longest.

In the 60's took a Military troop steamer from NYC to Bremerhaven - 8 days
returning 31 months later passage took 9 days.
Going over ocean was like glass not a ripple - returning gale force winds/seas
the 600 foot vessel tossed like a cork in a bath tub.

Long cruises with port stops are not all that bad but the T/O you better have
a big mega ship with lots of interesting things to do; gamble in the casino or
bring a bunch of books to read.

In the planning stage a voyage around the South America horn Buenos Aries EZE
to Santiago SCL (Valparaiso). A 14 day trip - early on there is a ship positioning
cruise from San Diego to Valaraiso - 17 days NCL SUN a very nice older ship.
The fun part is figuring out all the airfare/nonrev travel options - "Just get me to
the ship On Time" !

Day by day getting my feet wet or shooting at them here at this forum.

Name of the travel game Planes; Trains; Automobiles and DAM* Ships.
*Holland America names all their ships with a DAM (translation=City).