When this feature is enabled in your org, perform the following steps to set the expiry time on the form -
- Go to the About Update Client Details tab --> Step-6 --> Click on the Edit button.

- The following option occurs on the list -

- Set the expiry time in seconds, and click on Save & Close button.
- After the following steps, the expiry time will be set on the form you send.
P.S. Discover Update Client Details, the easy way to keep client information up to date in Salesforce. Find it on AppExchange - https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EcsATUAZ