Flight credits vary by airline, with expiration dates usually ranging from 12 to 24 months. Delta allows for combining up to five eCredits, offering flexibility when booking future flights. Check each airline's specific expiration and transferability terms to ensure flight credits do not go to waste. Most frequent fliers will find themselves with a flight credit or two at some point during their travels. Whether it’s from a canceled flight or a gift from an airline in exchange for getting bumped, there’s one major thing to keep in mind: You need to use it, or you may lose it. “Our members have been able to use credits to book flights, but many have also lost out on using these credits because it can be confusing sometimes,” Jesse Neugarten, travel expert and CEO of Dollar Flight Club , shared. Here’s what you need to know about flight credits, including major domestic airlines’ policies and helpful tips for using them.  What are flight credits?…