To the best of my knowledge, only the 1-month and 3-month subscriptions have automatic renewing. The 6-month does not. That's the one I use, personally, but I hate that it doesn't renew on its own - I like to just not think about having a lapse in service, go figure. For some reason that is the only subscription option on PS4 without the auto-renew function.
To keep from lapsing I just set a reminder on my phone for 6 months later on the day I manually renew. Hopefully they'll add this feature in the future.