Most clients I will have keep one for only a week or two. Others may have to keep one for years. The client will help you to dictate this based on their results AND their questions. A good food log can help trainers spot flaws in diet, but only if the client is honest. A weeks worth of honesty isn’t hard; however, twelve weeks has led to some absent food entries time and time again. Its supposed to be a tool, not a punishment. Ask your client their thoughts along the way when having them keep one.