Alternator belt life
Rule of thumb for a V belt drive for an alternator is about 100amps per belt, assuming the belt is in good shape and the pulleys are in alignment. As with most marine systems, keep an extra belt on board when the existing one is on the way out. Black powder around the engine, squealing sounds are clues that the belt is slipping and needs tightening or when the belt is too narrow, replacement.
A 65 Ford mustang that I drove had a worn alternator mount where the pulleys were not in alignment, it would squeal occasionally and was terrible on belts.
Cal 9.2 #19 SilverSwan