To check how long you've been subscribed to someone on YouTube, follow these steps:

1. Log in to your Google account on 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 and find the "YouTube history" section, then click on it.
5. Click on "Manage history" to open your YouTube activity page.
6. On the left side panel, click on "Interactions."
7. Scroll down and look for "Channel subscriptions" and click it.
8. You will see a list of all the channels you've subscribed to, with the dates when you first subscribed.

Clicking on a specific channel will show you the exact date you subscribed, allowing you to check how long you've been subscribed to that channel. This method works whether you are using a PC or mobile browser and does not depend on your watch history being intact. It taps into your Google account's activity log to reveal subscription dates.