My guess is it'll be around as the most up-to-date Windows on the market for about 4 years. It'll take some time to overtake Windows7 which it will inevitably do, and in much the same way that Windows7 has just done.
People make a big song and dance about WindowsXP having a longer life than other Windows' but people forget that Windows XP SP2 is COMPLETELY DIFFERENT to Windows XP vanilla. The original didn't have any wireless drivers (because wifi didn't exist), didn't have RAID, didn't have PAE (and therefore no DEP). It didn't support large disks or HD graphics or service isolation and it basically looked and felt like Windows 2000 with blue bubbles round the windows. It's easy to forget, particularly when it's convenient to your argument to do so.
Microsoft's progression of OS's is really much more like:
Windows 95 (lasted about 4 years)
Windows 98 (lasted about 4 years)
Windows 2000 (lasted about 4 years)
Windows XP vanilla (lasted about 4 years)
Windows XP SP2 (lasted about 4 years)
Windows Vista (lasted about 4 years)
Windows 7 (currently holding at 2 years)
Windows 8
Sometimes it feels like there is a large number of people on the internet who just yell "CHANGE! CHANGE! WE DON'T LIKE CHANGE!!!" at absolutely everything Microsoft does and it does get a bit tiring after a while.