No expiration date. Here are the T&C's fom the certificate that I purchased.

Gift Certificate Details
Recipient may use this Gift Certificate for purchasing air travel on AirTran Airways at AirTran.com, through our call center at 1-800-AIR-TRAN, or at the AirTran Airways airport ticket counter at any AirTran-served destination.
The Gift Certificate does not need to be presented at check-in for an AirTran flight. The Credit may be applied to any fare on any flight to any AirTran-served destination but is not refundable in whole or in part.
When Gift Certificate funds are applied to a ticket purchase, the fare rules for that ticket are applicable to all funds used for the purchase.
In cases where your total fare is less than the amount in the Gift Certificate account, the credit balance remaining will be held in the account until it is fully used.
This Gift Certificate has no cash value. It reflects the amount of Credit initially deposited in the AirTran electronic Gift Certificate account number referred to above.
The Gift Certificate account number above is a confidential number and should be protected by the holder of this Gift Certificate. AirTran is not responsible for theft of Credit amounts in Gift Certificate accounts.
Gift Certificates do not expire.
AirTran Airways reserves the right, and the holder or purchaser of a Gift Certificate agrees that, AirTran Airways may at any time change the terms of the program or contract with a third party to administer the Gift Certificate program.