Of course if we had our way everyone would need to change their will on a weekly basis. Sadly this is not true.
You really only need to change your will after certain key events in your life. But there's no reason to stop you doing it whenever you feel the urge.
Times when a change is advisable:
It's worth reviewing your Will at least every five to ten years. When you decide to make a change the easiest thing to do is to destroy all copies of the old one and write a new one. Small changes can be made via an additional document (a codicil), but it is much clearer to simply start again. Do not attempt to physically alter the original will.
You don't need to make a new Will when you, or someone mentioned in the Will, changes address. However you can make life easier for your executors by keeping a current list of addresses with your Will.