Yes, you need 12 consecutive payments. In your case, since you stopped your subscription in September and October 2017,. the count reset and you will need 12 months starting November 2017.
The perpetual license you can get after these 12 months mean that you will keep the last version available at that point.
If in November2018, the latest version of Substance Painter is 2018.3.x, you will have access to this version forever, but you won't get any new updates unless you start the subscription again or you buy a maintenance for Substance Painter, which will give you access to 12 m ore months of updates.