I would have a question about how I could resolve a user logging in to my website with Twitch, asking my app for permission to see which channels or in this case, see who is subscribed to my channel and how many months to subscribe ? Is this possible at all in any way?
Sounds like you want to get all channels a user is subscribed to. This is basically not supported. There is Reference | Twitch Developers but you need a pair of ID’s you can’t get “every” a user is subbed to.