(covers information from several alternate timelines)
This page contains information regarding new Star Trek material, and thus may contain spoilers.
"Space, the final frontier. These are the voyages of the starship Enterprise. Its five-year mission: to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before."
In the alternate reality, nearly a year after stardate 2259.55, the USS Enterprise, under the command of Captain Kirk, began the first five-year mission of deep space exploration into uncharted space that had ever been attempted by any starship. (Star Trek Into Darkness)
The 2265-2270 five-year mission of the USS Enterprise mission was chronicled in Star Trek: The Original Series and Star Trek: The Animated Series, based on the opening narration to those series. The Star Trek Encyclopedia and Star Trek Chronology have conjectured that Captain Robert April had one five-year mission and Captain Christopher Pike had two five-year missions with the Enterprise before Kirk assumed command. These sources also state that Kirk had a second five-year mission after. (Star Trek: The Motion Picture)
Until 2259 in the alternate reality seen in Star Trek and Star Trek Into Darkness, a five-year mission of deep space exploration was unprecedented. Because the alternate reality was a divergent timeline that split from the original universe in 2233, there presumably had not been five-year missions prior to, at least, that year in the prime reality.
No references have been made to five-year missions after the 23rd century. Instead, the opening narration of Star Trek: The Next Generation refers to the Enterprise's "continuing mission". However, in 2363, the USS Olympia embarked on a similary-named eight-year mission. In an interview in the DS9 Season 3 DVD, Ira Steven Behr said that the USS Defiant was "on a five-year mission to kick ass."