As has already been said, it depends on what you mean by "how long does each console have left on the market?"
I could be wrong but I expect that you will have Nintendo or Microsoft releasing a new console in 2011 or 2012, and the remaining manufacturers will release their next console the following year.
All three consoles have sold enough hardware that (as long as their next generation system is backwards compatible) I suspect there will still be games on store shelves for these systems until 2015 at the earliest; and the Wii might still have software in stores (mostly in bargain bins) for a few years after that.
While many people will probably disagree with me, I think that the PS3 has the greatest risk of rapidly declining hardware sales after the next generation of consoles is released. Unless the PS3 continues to see rapid large price reductions, the PS3 could still be a relatively expensive console which has less processing power, features and is simply “less cool” in comparison to the new consoles; and that makes the PS3 a difficult sell to a lot of consumers. Systems like the Playstation, PS2, Gameboy/Gameboy Color, and Gameboy Advance were able to see decent sales for several years after their successor was released in a large part because they were such cost-effective gaming; many of these systems could be bought for $100 (or less) and you could pick up a massive collection of games (both new and used) for $5 to $20 a piece.
