So have you inquired how often other boats in the marina, but NOT around you change their zincs. Is it a local or marina wide? Your marina should have a good idea how long zincs should last, and if your are not ask them to check the dock and the water for stray electricity. I have a diver check/clean the hull ever three months just of make sure nothing has change.
Also the boat with out zincs maybe using you zincs for protection? Causing the zincs on surrounding boat zincs to be used faster. I would assume the other boats and yours has a galvanic isolator protects the boat when plug into shore power. An isolator allows current to flow out of the boat, but NOT in using the green ground AC electrical wire which is connect to the green zinc loop wire. If not you should install one, and if you do have one make sure its working properly.
The Galvonic Isolator and the zinc protect loop is one of the important systems on the boat but the least understaood. I test for stray electricity most months, especially if there is a new boat close to us.
If the zincs re going to fast you could added additional/bigger zincs and/or change to aluminum. The last time we pulled I had a big Divers Dream zinc added in the bow to protect the bow thruster as the small clam shell had to be replace every 6 months. Also some boat hang a big grouper zinc over the side for additonal protection.
Anyway ask around and see what the marina norm is!