Wii Launched 2006

PS3 2006

X360 2005

18 months left for the Wii, 24 months for PS3, no idea about X360. That's what I think anyways.

18 months for the 360, 2 years for the Wii, 4 years for the PS3. I'm figuring in a staring contest between the 360 and Wii where Microsoft will blink first, and Sony playing catch-up having started development on its PS3 successor very late by industry standards.

The Wii will be like the PS2 and sell for years, but if you mean when the successor comes out I think it will go down like this:

Nintendo - 2 years from now
360/PS3 (depending on how long M$ pushes Natal)

The Wii will continue to sell like the ps2, the Xbox 360 will last more years.

Normally a system lasts 5 years on the market before a successor is brought into market. This Generation will last longer than the last (except for Wii) because of the rising costs of development and Sony/Microsoft pushing motion control add-ons for their systems.
I would guess:
2011-We'll see Wii 2
2012-We'll see Xbox #3
2012-We'll see PS4

I say..

Xbox 360 - 1 more year
PS3 - 2 more years
Wii - 3 or 4 more years, unless Nintendo cuts the lifespan short by releasing their next console earlier

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.

I would assume a couple more years for all of them. I think Nintendo and Sony will try to come out close together.

Before they stop being available first hand in shops in the USA

Wii 4-5 years
PS3 2-3 years
360 2 years