’Normal’ zincs should be replaced when they start looking pitted / rotten. If a boat sits full time in salt water in a ’hot’ marine, that might mean 2 months. It a boat is used only in mountain fresh water (like my dad’s was), then 20 years or more!
The loss of material is the real way to tell. Performance metals added the red dot system to help people with a wear indicator. Look at your anodes and judge the condition. If they are grey and have not been in the water for a while, scuff them with a scrub pad to make them shiny. Just like corroded electrical wire, cleaning it up makes for a better electrical contact. Better electrical contact with the anode to the engine and water means it will do the job better.