The Okudas started the ball rolling that five year missions in TOS might have been the norm. A 'standard' mission statement. They theorized that Captain April did a five year mission, and that Captain Pike did two (with a refit in between).
That was a common idea in fan sources long before the Okudas published anything. It was used in the 1980 Spaceflight Chronology
, for one thing. I'm pretty sure the conjectural chronology published in one of the Best of Trek
volumes (which drew heavily on the SFC) broke down Pike's time on the E into two 5-year missions. I did the same in my own personal chronology long before the Okudas did, most likely following the BoT precedent, although I'm no longer attached to the idea (see below).
But I've always been keen on the notion that maybe the Enterprise under Kirk was on a peculiar assignment. Maybe certain ships were given 5y missions into deep space, but others (including maybe a few we actually see in TOS) are on more specialized, short-term missions.
Indeed. Canonically, we have evidence for exactly one 5-year mission, and that evidence doesn't even come from TOS itself, since the main-title narration doesn't really count as in-universe evidence. (If the series had been a big hit and gone on six or eight years, you can bet they would've just ignored the whole "5-year mission" thing -- much like Run for Your Life
ignored its original "hero has 18 months to live" premise when it ran three years and M*A*S*H
glossed over the Korean War's 3-year span when it ran 11 years.) The only proof we have that 5-year missions were a real thing at all comes from Kirk's line in TMP, Icheb's summary of Kirk's mission in VGR: "Q2," and Star Trek Into Darkness
So I've never understood why so many fans assume that every
mission of every
starship is five years long. Not only can you not assume a pattern based on a single example, but it makes no sense as an assumption. It should be self-evident that different types of mission for different types of ship would have different durations. A 5-year general-purpose exploration and patrol tour would logically be just one category of starship mission, reserved for large, multipurpose ship classes that are capable of sustaining the demands of such an assignment. And presumably a ship on such a tour could be recalled or reassigned, or the tour extended to last longer, depending on the needs of the situation.