FMA - pretty soon. It's nice to see an extremely popular battle manga that actually knows how to finish. It seems almost every other shounen battle manga forgets that stories are supposed to end sometimes.
Naruto - It seems all the main villians have been revealed now and its somewhat clear who's going to be fighting in the final battle(s). But still, this is a shounen jump manga we're talking about so maybe 2 more years.
Bleach - The final battle is going on right now...but it started like over a year ago sooo I'm guessing Bleach has got like 3-4 more terrible, awful, ugly years left.
One Piece - Hopefully tomorrow because I hate OP with every fiber of my being, but seeing as how people seem to like terribleness it will probably go on for at least 10 more years.
Gantz - Final Phase started, but chapters come out bi-weekly and its always had a pretty slow pace. Phase 2 (~70 chapters) took 2-3 years so I'm guessing Phase 3 will take 3-4. Oku has stated that he knows how it will end so at least it won't drag on like Bleach.
Berserk - Should have ended with the golden age arc when it was still good. I've dropped it because its starting to remind me of Inuyasha (which I hate) with its monster-of-the-week kinda thing. So I really don't know whats going on at all right now. From what I hear its going to last forever so I give it 6 years.
Konan - 800 years