Unless you live in an especially dusty area I would say no more often than
yearly. You might want to just check it's operation every other oil change
for a little bit unitl you get a feel for when it might show signs of being
contaminated. This varies not just because of the environment that you're in
but also because each car's pcv and air filters vary so much in condition
and effectiveness and this leads directly to entry into the vacuum hoses.

Just push up on the piston and block off the vac port, if the piston falls
more than about halfway or so or just gradually creeps down, then it needs
service. If it remains up and snaps shut after you release the vac port,
then all is well. If it requires service you generally just need to wipe it
out and replace the grease with some mobil one synthetic bearing grease or
equivalent. If the piston, body, or o-rings are damaged let us know and we
can get you replacements.

Regards,

Angel Robles
US Director of Operations
www.forgemotorsport.com