The longevity of a sacrificial anode is a function of its weight. The amount of protection it provides depends on its surface area. When a zinc lasts less than a year, you need one with more weight. If the protected metal shows signs of corrosion despite the zinc, you need more surface area.
Check all anodes at least annually; if half (or more) of an anode has been lost to corrosion, replace it. Don't forget to check the zinc pencil inside your engine's heat exchanger; you will find it (or not) under a brass plug in the exchanger.