Re: How common are five year missions?
It seems pretty clear in the initial instance that the Enterprise was conceived of as being on a fairly normal tour of duty, that five years was a pretty standard length for a "patrol" of that nature. The notion of its mission being somehow "special" came AFAICS with all the later inflations of the Enterprise into the Most Important of All Ships and Kirk into the Galactic Gary Stu instead of just one ship captain among many.
"The real and lasting victories are those of peace, and not of war." - Emerson