I think it depends on the AP. I purchased mine in July but am not going until this weekend. When I bought mine, it was a special buy one year, get one year free promo, and it was activated automatically on Sept. 30 if not activated before then. So my two years started on Sept. 30, not when I actually pick it up.
According to the email I was sent once I bought the AP, I had to bring the credit card I paid with to guest services to get the AP.
I'm not sure if this is standard with every AP, but that's what happened to me.