My XP machine gets rebooted roughly once a week, but like folks have said, driver installations and major software installs/uninstalls sometimes require more often reboots than that.
As far as my uptime record is concerned, as an undergrad, I liked to see how long I could keep Instant Messenger online without the internet connection going dead and disconnecting me. (The T3 line at my school got a bit overloaded sometimes.) Since that would also require leaving the computer running without a reboot, I can definitely say I made it at least three weeks at one point... but unfortunately don't have an exact number.
Frankly, I've found XP to be Microsoft's most stable OS to date. I do have a beta copy of Vista installed on my machine as well, but I don't usually stay booted into it long enough to fully test its stability over time. Does anyone have any experience with long uptimes in Vista?