I have never insisted upon music practice, lessons, etc. for my kids. I've shown them things here and there, provided them with instruments and helpful books and that's it. I know some Montessori elementary schools have a set music time each week (I don't know if the children have the option of going or not) and have structured lessons. I have no clue what sort of expectations there are in terms of what the children do between lessons.
Let me say, though, that the Montessori approach works well even for music learning and a separate set-aside time is not necessary.
My daughter is nearly 13. Years ago, she was working through various kids' piano books, but let it go and as time went on, kind of felt like, "I've forgotten everything," and "I'm never going to be any good at it." She, for some reason, got hit by the piano bug again this past summer and started playing things at our old electronic keyboard (I've had it since I was 10 or 11!) I purchased a few new books and she made her way rapidly through the book for older beginners, feeling great as she went along. Every now and then, I did present little lessons that I hoped wouldn't come across as absolute corrections, even though they kind of were--like holding her hands and fingers properly. I didn't make a huge deal, just answered her questions as to why and took off the pressure by saying, "Don't really focus on it while you're trying to learn a song. If you try to do some little practice things with your hands curved, it'll go better." Taking the pressure off is always so important! Especially with this child! :)
In any case, her hand curvature is better and better and she is playing daily. Not because she has to, but because she wants to. Her progress since we purchased a new keyboard last month--which meant actually playing again because she had grown frustrated in August with the old crackly, inconsistent sound of the old keyboard--has been phenomenal. I offered to find piano lessons for her, but she replied with: "I don't want to have to practise and work on things that I don't really want to do. I just want to play and have fun with it." Piano lessons from outside source = pressure. She doesn't want the pressure. She places enough pressure on herself!
Will she ever be some virtuoso? I have no clue. Maybe one day she'll hit a point where she would like to improve beyond what she's been able to do on her own and then seek out someone to guide her further. But until then, the freedom to progress at her pace with the pieces she wants to learn... Well, I can't think of anything more Montessori than that, can you? :)