Can a crush on someone go away just like that?

Anonymous
This guy in one of my classes last year liked me, it was clear as day. He was constantly looking at me trying to catch my gaze, smiling, etc. He had never actually talked to me though. I found out later he had a girlfriend so I figured that was why nothing ever came of it. We messaged each other on Facebook over the summer here and there (he initiated). Eventually it stopped, more towards the end of the summer. He is still with this girl, seems more serious about her now. I've seen him a few times since the start of school, usually from afar so I don't know if he's seen me, we don't have any classes together.

I'm just wondering how long it has taken for your crush on someone to go away? and if having a girlfriend/boyfriend makes that crush completely go away? thanks :)
Updates
+1 y
btw, I don't mean my crush on him, I mean his crush on me. I'm kinda wondering if his thing for me has just gone away because he has a girlfriend. I'm trying to make as much sense as I can lol sorry I didn't clarify that.
Can a crush on someone go away just like that?
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