How many Product-ids to create for Subscription in

For your information, I have already implemented In-app Purchase in the application and it is working fine with Content Hosting at Apple as well as Server Model.I have logic in my app to check whether user has already downloaded the content ,if not download it using NKIssue as it is Newsstand app.

Here is the my requirement for Subscription : There is one issue of the magazine published each month. What I want is for users to subscribe i.e. for 3,6,12 months, and during this time, they receive one magazine issue each month as it is published. If they want to buy any of the previous issues, they would have to purchase those as an standalone in-app purchase.

Over here, I am getting confused about the Subscription while creating the Product ID's to include this feature. I have created n number of Product-ID for n number of Products in IAP but what will be the case in Subscription. I am bit confused here, about handling the Subscription part, Do I also need to create the n number of Product ID's to support Subscription.

For instance, If there are 12 magazines in a year, Do I have to create 12 separate Product IDs for subscription each of 3, 6 ,12 months (Total : 36 Product ID) OR creating a single product id for each of duration , irrelevant to product will work ? (Total : 3 Product ID)

similar to,

3 months : com.mycompanyname.pdfproduct.3months
6 months : com.mycompanyname.pdfproduct.6months
12 months : com.mycompanyname.pdfproduct.12months

Update 1 :

Which is the best approach to check, if the user is already subscribed for 6 months and unknowingly , trying to subscribe for 3 months subscription ? So, in that case, will app be allowing to subscribe or need to restrict the user for the same ?

How the app will check for this duration/span of time OR there is already any method/approach ?

Reference Tutorials , if that could be helpful for someone :

Introduction to In-App Purchases in iOS 6 Tutorial

In-App Purchase with an IBAction / Button

Thanks,

In iTunesConnect you must create one subscription family. Name it somehow. Then add 3 different durations for this family: 3, 6 and 12 month and give them ID's like you wrote above: com.myCompany.product.subscription.3months/6months/12months