The scope of the data is the utterance until it reaches a termination point and Siri thinks the user has stopped talking, along with information like the device model to tailor the response back to the device and a random device-generated identifier, which is the key to the data that is held by Siri for purposes of your interactions with Siri. This is an identifier that is separate from your Apple ID and not associated with any other account or services at Apple.
On May 29th, 2019. See this statement in context.