P&O Cruises, the British sister line to Princess Cruises, offers a couple cruises around the world out of Southampton, England, (round trip -- literally!) every year. Princess Cruises usually publicizes these voyages in the Captain's Circle newsletter, and your travel agent also can book them through Princess headquarters. These cruises typically last 90-100 days.
Of course, you also could follow in the footsteps of Mr. & Mrs. Joe Artz -- who book the Princess Suite aboard MV Royal Princess several cruises back to back a couple times each year. They actually spend over two hundred days per year cruising throughout the world aboard that one ship!