The answer will be different for everyone as there are several variables. Shooting from the hip here, I'd like to see some type of safety training done each month. If that's not possible during planting and harvest season....then drop out a few months at peak times. Why do I say that? I'd like to keep safety awareness going on throughout the year so safety becomes the culture. If it's possible, have group training and engage employees to share stories, make suggestions or be involved in resolving something that could be hazardous. Employees are more apt to support something they help build. FYI - OSHA requires annual training.