Hello, Phil
I moor at Queen City Yacht Club on Portage Bay, (for anybody not familiar with it, it is fresh- located between Lake Union and Lake Washington in Seattle).
I get about 18 months out of a hull zinc, and that seems to be typical of results experienced by neighboring boaters. I am going to switch to aluminum next time I change zincs. Most of my actual boating is done in salt water, but the boat (alas) spends a lot more days every year in her freshwater slip than out cruising. Zinc seems to have a very slight edge over aluminum in salt while aluminum seems to have a clear edge over zinc in fresh. Galvanic corrosion is a matter of time elapsed, not miles traveled, so beefing up the freshwater protection (without sacrificing the salt) makes sense in my case- as well as for many other boaters who moor in fresh but do most boating in salt.
I've always heard that it's best to replace a zinc when it has eroded to about 50% of its original size. Something to do with a relationship of the surface area of the anode to the items being protected.