Once the funds expire you can call and pay $100 per ticket to have the expiration extended for another 6 months.
NO That I not right what DVC California said. THe new ticket will take on the old expiration date it will not give you a new expiration date just because you booked a new flight!!
Any money from the credit applied to a new ticket will have the expiration date of the original ticket that has the credit. So if the orginal ticket was bought on say Sept 10, 2013 all future tickets will have an expiration date of Sept 10 , 2014.
What I do when I use the credit money is I purchase one way tickets. I use the credit funds to buy the outbound flight. Then I book the return flight and use my CC to pay for it. THe reason is that if for whatever reason you have to cancel the trip if you book a roundtrip ticket the credit funds and the extra money you had to pay toward the ticket, will all expire one year from the date of the orginal ticket with the credit IE: Sept 10, 2014. So its best to book two one ways that way the return ticket will at least have a different expiration date based upon when you purchased it. That way if for whatever reason you have to cancel and can not travel by the expiration date of your travel funds at least you won't loose all your money.