Every time your change your tire. Also, insist on a new valve stem cap. Here is a picture of mine that is less than a year old. I don't use nitrogen anymore as the local station free digital pump doesn't support nitrogen. Cheap caps or over tightening.
I've always been a believer in the metal valve caps. Up until my last two bikes at least one of the caps was the stem wrench tool as well. Very handy if you ever need it. I'm not sure what the change out interval is on the metal stems on our TPMS models. Agree with chopin114 about a new one on every tyre if you have rubber stems though.