Variation

There are no hard and fast rules for when you should drastically change your workouts. Much depends on how you react to your current training-program, your predisposition and your training-history.
Changing your training-program will stimulate the muscles differently, forcing them to adjust. Aside from a protein-rich diet, variation is the most important factor to ensure positive long-term development.
Once you have reached the intermediate level, you will notice that you exercise more often, require more food (proteins) and need more rest, whereas at the same time you will start to use various supplements.
It is important to start implementing in your daily workouts any extra information you may have obtained from books and magazines about exercising and nutrition.