I personally shock my well once a year to keep the bacteria count to a minimum but I have tested wells that have been ten years without a shock treatment and still had no health concerns. None the less I recommend a yearly treatment. The goal is to kill all the bacteria that will make you sick. Generally these bacteria include e-coli and coliform but there are several others. The shock should kill the bacteria but having the well tested a week after a shock treatment is a good idea just in case. I have written extensively on how to shock treat a well elsewhere on this blog so look around for that. Due to space constraints I recommend you go there to see the procedure I use.