At the end of a plan year, your employer may allow for up to $660 of unused funds to roll over into your account the following plan year.  

The rollover for a flexible spending account (FSA) is not cumulative. At the end of each plan year, up to $660 of unused funds may be carried over for use in the next plan year. The rollover amount can never exceed $660. Any unused funds in excess of $660 at the end of the plan year will be forfeited.