Well, I'd say it can go on for quite a while. Just to tie up the plot threads that exist right now (assuming no more are introduced), we have:

1. Sasuke and Oro / possible Sasuke-Naruto reconciliation
2. Akatsuki
3. Tailed beasts / giant demon (probably tied closely to 2)
4. Anbu / Roots motives in Konoha
5. Naruto becoming kage

I would assume 1 will take 50 chapters at least, 2 and 3 together might take 100, 4 less than 50, and then naruto has to do a lot of training to become a much better ninja in order to be Hokage - so I'll allow at least 50 chapters for number 5. Knowing that Kishi likes to draw out things, then I would conservatively estimate 250 more chapters. This would make it quite a long running manga, but One Piece might have it beat since Oda said somewhere he is going for 600 chapters at least.

@Chet - I wouldn't say Oro is stronger than everyone in Akatsuki besides Itachi. Kisame has large chakra capacity as well as strong techniques, and Deidara is no pushover either. Plus we haven't even seen Zetsu in action yet... I doubt Oro is stronger than them, he is merely on the same level. There have been other people using very strange techniques, so I wouldn't assume Oro has a hidden power just because he can do some strange things. Deidara has little mouths in his hands for crying out loud. Zetsu seems to be half plant. There is no way Oro can take on more than one Akatsuki at a time, so I believe he'll mostly fade into the background until the Akatsuki plot thread gets tied up.