Your unused credits will never expire. They are yours for life! Even if your account is paused or canceled, you will still retain your credits and can continue to use them at your own pace.
You can also use your unused membership credits to gift audiobooks to others. When you've found the book you'd like to gift, choose the button labeled "Gift Audiobook" which appears beneath the book cover.
From there, you'll be able to apply a membership credit to gift this audiobook in your cart.