That is not how it works.If you've paid the 13th month then presumably you're signing up for another year and you'll get your next one after only 11 payments.
And I spend 60 minutes on the phone with Apple to clarify this. If you wait 13 months, then you are one month short, if you want to get the next iPhone at launch, you have to pay the extra month to be eligible for it at launch, otherwise you have to wait until a month after it is released.