Apple recently revealed the fact that they keep all Siri inquiries stored on their servers for up to two years. That's right, every question, comment or message spoken to the personal assistant gets anonymized and archived to improve the service, according to Apple spokesperson Trudy Muller.
Here's how the process works, according to Muller. When you interact with Siri, the audio recording is beamed over to Apple's data farm for analysis. Apple creates a random number to associate with a particular user and their submissions. This unique identifier, which isn't your Apple ID or your e-mail address, represents the user's Siri submissions with regard to back-end analysis.
After six months, the voice clip in question is disassociated with the user. The clip is then stored for up to 18 months to assist Apple in testing and improving the product. It's worth pointing out that if a user turns Siri off, both identifiers are deleted immediately along with any associated data.