The simple answer is to update your will any time there is a change in your beneficiaries, representatives, estate assets or the tax law. The following article lists some specific dates to look for- if your will predates the listed dates you may want to come in for a technical update even if your situation has not changed.
-Dan
http://www.marketwatch.com/story/check-the-freshness-date-on-your-will-2015-06-16