Apple has confirmed that it keeps Siri-related data for up to two years after the information is anonymized.
According to Apple spokesperson Trudy Muller, Cupertino takes the issue of privacy very seriously, and the anonymized data is collected for the sole purpose of improving Siri as a service.
Indeed, the majority of work on Siri is executed remotely, which is why the software requires an open Internet connection to operate.
In any case, voice clips stored by Cupertino are apparently categorized by random numbers, which are not associated with an Apple ID or email address.
After 6 months, the random number is no longer associated with a saved clip, although the audio file could be saved for up to two years for "testing and product improvement purposes."
However, if a user disables Siri, the randomized identifier is deleted, along with any related data.
As AppleInsider notes, the fact that Siri data is sent to Apple before it can provide results has been a concern for security advocates and various companies. Indeed, IBM has apparently banned Siri on its corporate networks.