You have 6 months after maturity to cash them out. This info is from U.S. treasury website. The treasury website allows you to put in bond serial number and tells one when they mature.
In 1992 I won 3rd prize in a Kimberly-Clark sweepstakes. My prize was a $10,000 savings bond which cost the sponsor $5,000. The bond finally reached full maturity and I cashed it out today. I received $16,500. Awesome prize!