Note class for managing information on various musical notes
Although you put the three classes into one file, they are already three classes. So you already split the responsibility. Translating a float to a length is one "responsibility" and translating the pitch to a name is another. Seen from this angle everything is fine.

SRP is a heuristic for good OO code. You won't go straight to hell if your class does two things. But if your class does 20 things you should consider a refactoring.

If you have the feeling that your code is not readable or your file is too long, just extract the inner enum to standalone enum. In most cases it is no difference if you have an inner class/enum or a real one, so you should prefer the more readable solution.

In addition to that, I would store the length as the enum at the note and translate it once in the setter (reused in the constructor) instead doing it every time in the getter. I'm also in doubt if I would offer an interface with a integer length getter and the enum length getter. Usually you want to force the caller to use the enum object, I guess.

You should try to use this and the getters in a consistent way.

Have you considered all the nasty pitfalls in using the float type? If you don't want to do any math I would prefer String, otherwise you might want to think about using BigDecimal.