I believe you have a year to extend the credit, and then it is good for 6 months, so the timing of when you request the credit can be important. I have done it several times (used to be $75, not $100). If the same person has a couple of credits, combine them before they expire into one reservation number. (book a flight, apply the credits, pay any small difference, cancel it quickly, you get any difference back in cash, and now have one reservation number; warning it will expire on the earliest date of the ones you combined)
What is nice is that the extended credit is a LUV voucher, usable by anyone, no longer restricted just to the original person. (I had a credit in my daughters name, she cancelled out on a family trip after I bought the plane ticket, so I was able to use the LUV voucher credit.)