According to their online member guide, in the section "Membership Qualification Year":

"Elevate Members must earn sufficient status Points within the Membership Qualification year to achieve or retain Elevate Silver or Elevate Gold Status.
The Elevate Status qualification year is based on a calendar year and runs from January 1 to December 31 each year."

The points you can use to book flights don't expire with any activity that adds to your available points balance (these expire after 18 months with inactivity), but you have to earn and retain enough tier status points every calendar year.