My best guess is that having experience with PCB design is 80%, and knowing which buttons to push in your software is the other 20%.
For the schematic design:
* Knowing how do draw a comprehensible schematic.
* Balance between connecting with wires, or using labels (and that horrible boxes drawing that is popular lately).
* Knowing electronics in the first place.
* How to handle simulators, knowing their strong points and their limits.
For the PCB.
* Footprint placement for optimal design.
* GND planes, GND planes and more GND planes.
* How to design for EMC.
* Signal integrity, differential pairs, crosstalk, separating digital / analog stuff.
* Probably many more subjects...
If you already know how to design schematics / PCB's then getting to know another PCB design program is probably a few days to weeks to get the basics, and a few months to get really up to speed and productive. But it also depends on your learning tempo, how much effort you put into it and how complex and user friendly the software is.
For the rest, I'm not qualified to advise you. I am very happy with KiCad myself, but it's also the only modern PCB design suite I know. Most others don't even run on Linux, and that's an immediate show stopper for me. KiCad is getting more widely used in the last few years. When autodesk bought eagle, a lot of the users of the "free" version switched to KiCad (and from what I hear from them, KiCad is a lot better then the old eagle). I've also heard of some people switching from altium to KiCad because KiCad is "good enough" and they find it hard to justify the cost of altium (Imagine companies who do maybe one or 2 PCB's per year to put in their own products).
I'm not sure how to interpret your question about KiCad. Do you want to know whether it's worth learning KiCad, or how to start learning KiCad?
There is:
https://docs.kicad.org/8.0/en/getting_started_in_kicad/getting_started_in_kicad.htmlThere are lots of tutorials on youtube, and there are complete courses on those "courses / learning sites".
KiCad also has a very active user forum.