More than a year later, on April 19, 2013 an Apple spokesperson responded to Wired that Apple “may keep anonymized Siri data for up to two years,” but Apple:
...takes steps to ensure that the data is anonymized and only collects the Siri voice clips in order to improve Siri itself.
...If a user turns Siri off, both identifiers are deleted immediately along with any associated data.”
Wired went to explain:
Here’s what happens. Whenever you speak into Apple’s voice activated personal digital assistant, it ships it off to Apple’s data farm for analysis. Apple generates a random numbers to represent the user and it associates the voice files with that number. This number — not your Apple user ID or email address — represents you as far as Siri’s back-end voice analysis system is concerned.
Once the voice recording is six months old, Apple “disassociates” your user number from the clip, deleting the number from the voice file. But it keeps these disassociated files for up to 18 more months for testing and product improvement purposes.