Just so people are aware though, the created_at field is the timestamp of when that subscription object was created, it may not accurately show when a user originally subscribed to the channel. For example, I have a channel where the created_at value is about 18 months ago, yet I’ve been subscribed to that channel for over 26 months. The reason for the inconsistency in my case was a change of payment method, but other things like changing sub tier can also affect the results.
Hopefully Helix will have a better channel subscriptions endpoint.