A Membership Account may be closed at the Company's discretion if no Points or Miles are accrued during a 24-month period. All Points in the Membership Account will be forfeited at that time.
From the web site terms and conditions.
That being said...points that you earn keep building up if not used. In other words, if you earn points over 10 years of membership, all points will stay in your account until used. The points don't expire, but as the T & C states, your account could be closed.