I have mine set to "on" and I never really mess with it. I don't know if it changes or not to be honest, it always looks about the same, in any lighting environment. I don't know if that means it's working or not 😛
One thing that I do notice is that every so often, I'll unlock it and it will be particularly dim. If I use it for a few minutes it will eventually brighten up (instantly (not over time)), or, if I relock and unlock again, it will be brighter.