How long has the 'hey siri' function worked unplugged?

That's not the "Hey, Siri" function. The "Hey, Siri" is when you activate Sir without ever touching the Home button and that only works when the phone is plugged in. The behavior your describing is the way Siri has always worked and doesn't require that you say, "Hey, Siri".

simonp_au wrote:

Ok, well after I press the home button as usual (unplugged), then ask siri a question, siri then answers. That's the way it has always worked, yes. But, if you leave the siri screen open, while unplugged, then say 'hey siri' siri activates again. Try it. And that has not always been an available function.

I can confirm that it works. My recollection is that Siri always remained open for a minute or so after a query, but you would have to push HOME again for another question. Nice feature!

Also worth noting that the "Hey Siri" handsfree feature needs to be active. Settings / General / Siri / Allow "Hey Siri"

I was able to replicate this action. I unplugged my iPad, started Siri, asked a question, got the answer, then with Siri still open, asked a second question just by starting with "hey Siri". I did not have to touch the screen for the second question.