It doesn't need to be "beginning to end", tankoubons often have multi-chapter stories along with other short stories to fill out the pages. I'll dump examples as I find them.
Story arc could also include a multiple short stories relate to the same characters that may involve many more characters to build tension/plot.
I missed the Korean story arcs that last thousands of chapters and never got finished
a story arc in my opinion isnt just because it has a long narrative writing where it consists of chapters and a few hundred or even up to a thousand pages but the plot and changes within its story or added works could also signify it
You're right that "long" can be subjective. By "long" I mean one continuous story with plot and the same main characters throughout, rather than one-shots or multiple unrelated short stories. It doesn't always have to be multi-chapter like #607102|Elf no haha to haramu made + afterstory|18,504 or #397218|Kko to Yamioji Raku | Lady K & The Sick…|93,533 although all multi-chapter w/ same char and plot would be "story arc".
As long as it's one cohesive story, an ongoing narrative can certainly use this tag.
I love a good story arc, and having a definition will help improve tagging. "Long" seems vague. I think the definition should give some guidance by including a minimum number of pages (or chapters/volumes) , although what the minimum should be is hard to say.
Can an ongoing narrative use this tag or is the tag only appropriate if a narrative story arc has ended?
yes "story arc" can be considered a narrower/child tag to "multi-work series". By definition, a work can either be "story arc" without being a "multi-work series", or both a "story arc" and a "multi-work series" (same continuous story split into several works/chapters).