The school district I used to work at, and still keep in touch with has over 2,000 workstations on XP and they have no plans of upgrading anytime soon. They stayed on Win95 until around 2001/2002 and didn't upgrade to XP until around 2004. When I got laid off, they still had around 200 machines running Win2000, but they have since replaced them with more XP machines.
The real answer depends on what you want to do; basic stuff should be fine, but if you have to keep up with the bells & whistles on the Internet you may discover that software support will start to drop off. But, with the huge numbers of people still using XP it could be a while. It sounds like MS is going to drop off XP support for its software sooner rather than later; probably trying to "force" people to upgrade, though third-party software developers will probably still be supporting XP for a long time.
