When do we consider the end of a generation?
When the first next generation console comes out? Then probably 2nd
When the last current generation consoles ceases production? Then I'll say 1st
I think the PS3 will stick around the longest, eventually outselling Wii in the weekly charts.
I think the Wii will peak and then drop suddenly, unless Nintendo changes their game plan.
I think the X-box will fizzle out slowly but maintain decent sales.
You may laugh, but the problem with the "mainstream" is that they are fickle and can't stay interested in anything for too long. Today Wii is king, but next week they might be running out to buy a furry red doll that laughs when you poke it. I am not calling the Wii a fad (yes I am, but being a fad isn't bad). It will maintain a dedicated fanbase like all Nintendo consoles, but until Nintendo tries to capture "hardcore" gamers again it will have to deal with a second place finish again.
This is only my opinion.