For the boat we used to have, I had to change the zincs once a year, and they were usually pretty degraded by that time. It was in salt water 24/7.
For this application, depending on the size of the zincs used, if that is the mechanism for protection, I would start with every six months to get a record of degredation, and modify the time as needed once a history is developed.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask