No more than once or twice a year dependong on how often you smoke the pipe. I was just talking to John at the Briary recently and he doesnt do it more than twice with once being the norm.
Sometimes you will smoke something that is just gross and it wont leave your pipe. Thats always a reason to get out some cotton ball's and alcohol...
I run a pipe cleaner though the stem and shank after every bowl I smoke. It I detect the faintest sourness in the pipe, I'll use Bee Sweet cleaner or Everclear to clean the stem and shank only.