The expiration date for your credits is one year from the Subscription Book charge date if your subscription is active. If your subscription is cancelled and no longer active, you will have 90 days to redeem your available credits when the subscription is cancelled.
To see when your credits expire, follow the below steps for both the iOS and Android Chatbooks Apps:
Tap into your Subscription.
Tap into the Gear Setting's icon in the upper right-hand corner.
Tap into "Subscription book credits"
There you will see your available credits and when they expire.
Here's a video how-to on finding your available credits as well:
How to find expiring subscription credits.