For me, it was so long ago that I can't remember. :P I'm guessing it took at least a few weeks for young me.
One thing I do remember is that I wanted to leave an angry comment on the CoTW quiz asking where Scotland was, but gave up when I saw I would have to create an account. Saved me the embarrassment.
I think back in 2020 and 2021 I knew a "countries song" of some sort that listed off all the world's countries, and I used that to ace the quiz, but forgot it soon after. I believe it took about a week to memorize the song and ace COTW.
I had already learnt most of the countries via different sources, mainly Sporcle which I used for a year or so before I found JP.
oceania and n.a were last
but there‘s a difference between knowing the names and knowing the locations
I can't remember when I learnt all the countries, but for some reason I keep forgetting Belgium.