Personally, I change my exercises every 6 to 8 weeks. Arnold for example, would change his exercise program every 3-6 months. You are right, because everyone is different and you have to find out what works best for you and your physique. Arnold did mention that changing his weight training routine stimulated new growth. He is absolutely right.
Muscles get stale and bored after repetitive exercises.
In the long run, new movements for each muscle group will stimulate growth. For example, if you do flat bench press for your chest routine, try to change it using dumbbells instead. After a few weeks, you will feel your chest explode . Varying your routine will allow you to find a whole range of different exercises . It will give you a better idea as to which exercises work best for you and which ones don't really suit you.