To get bigger and stronger, you need to stimulate your muscles through exercise. It's not so much the exercise that makes your muscles grow (although exercise is needed), it's the change from doing nothing to forcing your muscles to work that causes them to respond.
If you keep doing the same workout over and over again without changing, you won't make any gains. You won't improve your mile times or your bench press. This is because your muscles get used to what you throw at them and stop responding. Muscles react to change by growing stronger. It's a good idea to look over your workout every six weeks.
Reaching a plateau in a certain exercise can mean it's time to replace it with something new. For example you may be trying to improve your bench press. You've been benching for about a month and have seen a steady increase in the amount of weight you can lift. During the next two weeks, you begin to notice that you haven't been able to get over a certain weight and are actually starting to go down. If this is happening to you, you need to change something.
If you're using dumbbells, move to barbells. If you're starting off with flat bench press, switch it to incline bench press. The amount of ways you can change your workout is almost endless. Change is what helps you get to the next level.
The Bottom Line
You should switch out exercises about every four to six weeks or whenever you feel like you aren't improving anymore. This new shock will jump start your muscles into adapting and getting even stronger.