What determines the completeness of a comic book story arc of any length?
This is the basic standard for any story. And it can be done in 4 pages, 8 pages, 22 pages, 4 issues, 6 issues, or 100 issues, but that's the concrete you mix to set a story.
What isn't a complete story, again in any format, from TV to comics to books, is a soap opera-like meandering in and out of the lives of characters without any real plot development or growth arc for the character(s) going through the experience.
(Yes, massive, marketing-driven crossovers, I'm talking about you.)