darwinTouchIDAuthenticationAllowableReuseDuration property
If the user unlocks the device using Touch ID within the specified time interval, then authentication for the receiver succeeds automatically, without prompting the user for Touch ID. This bypasses a scenario where the user unlocks the device and then is almost immediately prompted for another fingerprint. and https://developer.apple.com/documentation/localauthentication/accessing_keychain_items_with_face_id_or_touch_id Note that this grace period applies specifically to device unlock with Touch ID, not keychain retrieval authentications
If you want to avoid requiring authentication after a successful keychain retrieval see darwinTouchIDAuthenticationForceReuseContextDuration
Implementation
final Duration? darwinTouchIDAuthenticationAllowableReuseDuration;