What's the maximum level for each skill?
And will it take long to maximise them
I hear everywhere it's five. Long or not, that depends of your own dedication, and how much available time you have in your hands each day. The reasoning is you can fly down the tree at level one if you want an overview, then review each skill to level two (or three, or whatever you like), or have each skill at level five before moving on, or any mix.
So I wouldn't think in terms of "how long", rather "how many practices", and it's actually better for long term retention to let time pass between repetitions, and to spend actual months on the material.
More when I'm switched, viel Glück :)
https://support.duolingo.com/hc/en-us/articles/360002254671-Crowns-FAQ I think this answers everyones questions