The DEK is cached in memory for several read/write operations and evicted from memory at a regular interval such that new requests require another request to your cloud service’s key management system. If you delete or revoke your key, reading or writing to notebooks fails at the end of the cache time interval."

The timeout is set to 30 mins.

Doc link - https://docs.databricks.com/security/keys/customer-managed-keys-notebook-aws.html