Why would a 1-time payment be a subscription? Wouldn't it be a one-time purchase? Yes there is a difference.
I'm not sure I understand your question, but I can tell you what I see for monthly subscriptions. When roChannelStore is queried with GetPurchases, subscription purchases return three values:
purchaseDate - the date the subscription was originally purchased; this never changes
expirationDate - the date the subscription expires; gets updated every month when it's renewed
renewalDate - the date the subscription will be automatically renewed unless canceled by the user
In general expirationDate and renewalDate have the same value. If the subscription is cancelled, renewalDate will not be present in the returned data. Once the expirationDate is passed, the entire purchase will disappear from the returned data.
This is by no means an official explanation; just my observation.
There are no refunds, so if you cancel, you still have access to the channel until it expires. And you can't sign up again as long as the subscription is still valid. After it expires you can sign up again if you like. And like Renojim says - if the user doesn't cancel, then it automatically renews.