Unused Store Credit remains in the customer's account until they decide to use it. The Store Credit does not expire unless you have configured specific expiration rules using Magento 2 Store Credit Extension.

Read this How to Configure Store Credit Extension in Magento 2 Store to setup this extension.

Here are the few more resource to help you get most out of this Magento 2 Store Credit Extension.

• Maximizing revenue with Magento 2 Store Credit Extension
• Raise Loyalty & Returning Visitors with Magento 2 Store Credit Extension
• Store Credit in eCommerce: A Key to Customer Retention