Robert McMillan for Wired:
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.
“Apple may keep anonymized Siri data for up to two years,” [Apple spokeswoman Trudy] Muller says “If a user turns Siri off, both identifiers are deleted immediately along with any associated data.”
Seems reasonable.