Long story short: Every 2-3 years you should sit down with an attorney and revisit your Estate Plan. You may think that's too often, but life and circumstances change frequently - you have children, you inherit some wealth from a family member, you get married/divorced, family moves away, your income drops off, etc.
There are a variety of reasons why your Estate Plan may be out-of-date for your current situation, but regular revisions can keep your Estate Plan current and correct.
Two of the most common reasons for amendments to an Estate Plan are: (1) Family situations changes - i.e., marriage/separation/divorce, a new baby, etc; and (2) a falling out with someone you appointed to act on your behalf.
Maybe you appointed your best friend as your healthcare agent and two years later she stabs you in the back - figuratively, not literally. She may not be the best choice for healthcare agent any longer.
Maybe you got divorced and want all of your assets to go to your children instead of your ex.
Regular reviews of your Estate Plan are required to maintain a current and correct plan.