I don't think you can measure it at all. I'm sure there are a LOT of people that can recite a load of quotes from Harry Potter, or any franchise, it doesn't mean they all love it as much as each other.
I can quote a lot of lines from Star Wars because I used to watch it all the time when I was about 8. I still remember those lines, but that doesn't make me a big fan, because I don't even care for Star Wars any more.
I mean there are some things I LOVE, and I mean really love. It's not music, and I'm not telling you what they are, I don't own any memorabilia because I don't have money I can just waste on novelty items, I don't know any quotes because I have bad memory, and only about two people know how much I can obsess over some things. It doesn't make me less of a fan than anyone else just because I'm not mentioning it every 10 seconds or plastering my walls in posters.