If you are referring to Savings Bonds:
Retention of Records
Retain copies of order forms and associated batch transmittals for four months in case a batch is lost or there is a discrepancy with order information. Also ask purchasers to retain their copy of the purchase order form until the bonds are received by their intended recipient.
A complete guide to savings bonds can be found here:
http://www.treasurydirect.gov/instit/savbond/guide/guide_sbr.htm