How long do I have to re-use an unused ticket credit?

The answer is almost always “within one year from something”. What that “something” is depends on the airline, and in some cases, the specific fare rules. Most domestic airlines require that the new ticket be bought and paid for within one year from the date the original ticket was purchased, though the new travel date can occur after the one-year mark. Other airlines require that the new travel dates occur before the one year anniversary of the original ticket purchase. Some use the dates of original travel instead of date of original purchase when calculating the one year mark. And there are a few other variations as well. If the unused ticket was itself an exchange of an earlier ticket some airlines will use the date of the exchange to calculate validity and some will return to the date the original ticket was purchased. For the details of your specific ticket, please check with your agent.