Just so you know, when she's in the Tower, it's described as a 'tight cap of curls' and she later thinks of it as quite long when it 'just brushes her collar'.

As Min's overall presentation of appearance has always been very important to her, I would imagine she wouldn't keep it much longer than this. Just because a description of a haircut has never been mentioned in the story doesn't mean that they've never gone to the bathroom either. Matt's hair is still short, but when was his last haircut? It's a boring, mundane activity that RJ obviously didn't feel the need to cover.

I would say that Min is described quite a bit, more than many others in the story. Others even adopt her style of dress, so that gets described over and over and over...I think there's more than enough to form a mental image and form an opinion as well.