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)