This is how I view it. I consider myself a Garen main, even though I play more games as non-Garen champions than as Garen.
Garen is my safety pick. I know exactly what I can do with him, and I know his matchups very well. I do like playing other champions, but when I'm thrown into an unfamiliar situation I don't reflexively reach for them. He's a constant that helps me focus on other parts of the game while I play.
It's not about how much you play a champion, but rather how much mental space you're willing to save for them. When I play Garen I hardly have to actively make decisions anymore. I just know where I should be, who I should try fighting, who I should avoid, and what I should be doing at each stage of the game. I make mistakes of course, but I don't have anywhere near this level of knowledge for any other champion.
On top of that, if I'm playing and just want to have a good time Garen is the champion I play. He's the champion I've played for five seasons and still haven't gotten tired of. I think this part here is the most important. To really have a main you have to be willing to play that character. It doesn't matter how often you play them, you just always have to be willing. I know more than a few people who say they "main" X champion, and next week they main Y champion, and they just switch around because they get tired of playing whichever champion they picked.
To truly main a champion, in my opinion, you just have to never get tired of them, no matter how much you play.