We change ours every 2-3 years. When you start seeing a space in the bushing or it starts elongatig the hole change it. Easy and cheap security. Rather do it in a shop in the winter time than in the field at harvest. HTH
Check thickness of bushing and replace when getting thin. Be sure to loctite new pins in and get them tight. Many people replace pins and they start breaking. They break when they have not been tightened properly. If not tight enough they will flex in the threads and start breaking off one by one. You best use the special socket to torque them in not a Cresent or end wrench.