I am also in SF (South Bay).
Spinning the prop does not help - strange but true. The barnacles grow regardless.
Over summer, as the water gets warmer, you need to clean the prop/bottom about every month. If you don't race, and don't mind knotmeter and depthsounder failing, a couple of months. Over winter, 2 or 3 months wait is fine.
I clean every month May-October, and a couple of times over winter.
To prevent major fouling and bottom cleaning, either clean regularly, or go spend a week in the Delta. The fresh water kills off all those saltwater critters!