You can see how long you've been subscribed to someone on YouTube by checking your YouTube subscription history in your Google account. Here are the steps:

1. Log in to your Google account in a web browser.
2. Click on your profile icon in the top right corner and select "Manage your Google account."
3. Go to the "Data and privacy" tab on the left panel.
4. Scroll down to find the "YouTube history" section and click on it.
5. Click on "Manage history" to see all your YouTube activity.
6. On the left panel, click "Interactions," then scroll down to "Channel subscriptions" and click it.
7. This will display a list of all channels you've subscribed to over time.
8. Click on any channel to see the exact date you first subscribed to it.

This method provides the precise date when you started your subscription to a particular YouTube channel, giving you a clear idea of how long you have been subscribed.