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).
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.
In TMP, when Kirk talks about "five years out there, dealing with unknowns like this" he makes it sound like not every Starfleet captain is as privileged as he.
(Of course, he was justifying his retaking command of the Enterprise to the neophyte Decker, who probably hadn't been too far into deep space before.)
Star Trek Into Darkness certainly seems to imply that a five year mission is a theoretical possibility where no crew has gone before.
(Yeah I know, altered timeline.)
Certainly by TNG, deep space missions were assumed to be truly long term prospects (more than five years, some sources even suggest upwards of 20!). Hence why Enterprise-D was fitted to accomodate families. She was supposed to be away from regular Starfleet for extended periods. Of course, this was seldom paid lip service on screen apart from a few moments in seasons one and two.