OP has been going for a long time, 469 (i think that's right) chapters...but i have to say, i don't think it's much of an issue. I love the series and i don't mind oda keeping it alive. Actually, i heard something once that oda had planned out the end of the series and everything, but it was so popular that he kept writing and writing and writing.
And that gives us today's one piece. the only arc i had issues with was sky island. it did advance the plot but it seemed like oda could have done it in a less tedious fashion. (edit out the hotori/kotori bit)
I think its on the edge of being too long. I like that it takes its time to explain everbodys background and so forth, and that everybody have a goal they must accomplish. But...the story is movign toooo slowly forward, there are too many "mini bosses" in my opinion..
I love the manga, funny as hell, but c'mon..get on with the story! and not so much fighting at the same time...takes forever when we have to see everybody in the crew fight somebody at the same time >_>
I'll just add my simple rhetorical opinion/question: wasn't it always the point, that this journey was to last a lifetime?