Found this on a US thread. It is from an official Forge representative.
"Unless you live in an especially dusty area I would say no more often than yearly or around the 30k mark and that's just to be safe. If you like to tinker then 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."
So it looks like we will be looking to service the DV intermittently.
