While cleaning the hull and shaft of my boat yesterday I noticed the shaft zinc was (it appeared to be) in good condition. I haven't replaced it in over a year and it's round and doesn't appear to be missing more than 1/3rd of its total mass. Is there a time limit zincs should be left on even if in good condition? My heat exchanger is a whole other story, every 30-45 days its worn to a nub but I understand that is not far from normal for a boat that is used frequently.