With this website you can find out when you subscribed which YouTube channel.
1 Your channel
2 Make your subscriptions public
Go to your Privacy Settings and uncheck "Keep all my subscriptions private".
You can undo it again as soon as you've finished using this website.
Here you can check how long you've been subscribed to a YouTube channel. YouTube.com doesn't offer a way to show the time you clicked the subscribe button on their website, but for developers, they have an API which returns a list of all subscriptions - with the date you subscribed it!
I made this website because I wanted to know for how long I'm subscribed to my channels and what my first subscription was, but I couldn't find a way to do it. Then I found the way in the API and created this page so everyone can easily use it.