One Piece can be expected to end in about 8 to 13 years from now. It's not going anywhere for a while. Naruto, well, I heard that Kishimoto said at last year's Jump Festa that the story had about two years left, and would end at around 500 chapters. That gives Naruto 1.5-2 years left, which seems about right. I figure that there's nothing left to do but have the final clash with the Akatsuki, which will result in Naruto confronting Sasuke and resolving that issue. Tsunade dies some time along the way, and Naruto becomes the 6th Hokage (despite being totally undeserving of the position), and he, Sakura, and Sasuke go down in history as the next generation of Sannin. Series over, and all that can be wrapped up in about 2 years. Unlike some manga-ka, Kishimoto does not have a tendency to extend his manga long past where it REALLY should have ended, for the sake of both the story and the readers. He's just shown a tendency to make the story suck while it was still going on.
Kubo, on the other hand, has been over-extending Bleach ever since he didn't end it in Soul Society, to the point where I have no idea what's happening anymore, because nothing's actually happened since Turn Back the Pendulum. The chapters simply aren't worth remembering. If Kubo doesn't intend to end Bleach after this arc (which could still take 7 years, considering Kubo's pacing), then I think Bleach may get "Shaman King-ed"- cancelled even after becoming a well established series with over 30 volumes, simply because it got SO unpopular rather late in the story. If Bleach continues as it is now without ending, I predict that it will last 2-3 years, maybe 4 for its past grace to wear out, before it gets cancelled. It's already beginning to lose its "Big 3" spot to Toriko and Reborn, at least in the Table of Contents, if not in tankobon sales, and if Reborn starts to pick up a bit, and Toriko gets more established... Bleach could be seriously dethroned.
In short, the current Big 3 (there are always really a few really big series in Weekly Jump, just all three of them don't usually last this long) will be around for at least another 2-3 years, at which point Naruto and Bleach can be expected to end/be cancelled. Toriko and another new comer series, along with One Piece, should be the new Big 3. Unless One Piece goes beyond rankings, like Kochikame, and a new series replaces it, but I think that the story is too connnected, popular, and well-selling for that to happen. I think it will retain its flagship title status for the duration of the series.