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 "Send as gift" which appears beneath the book cover. You'll be asked to enter your recipient details, delivery preferences, and a gift message. You can then apply a credit during checkout to zero out the cost.