Hi!
I'm doing a collab with an artist, so I don't know entirely how much time we are spending all in all, but I know I've spend around 50 hours+ on the script alone for a 5 episode webtoon.
So for simplicity, let's say 10 hours on the script.
Then we spend around 5 hours on the draft.
My artist spend around 20 hours on the lineart
And then around 20 hours on the color, I think.
So all in all, around 55 hours on a single episode.
So I would say, depending on how much time you use a day on making it, 3 weeks doesn't sound like too much.
But I would also say it depends a lot of your length and the panels and how much detail goes into those.
Also, it get's easier as you get more used to it, since making webtoons and such requires a lot of work, which aren't that obvious before you start making it xD
A tip is to be as organized as possible and work on layers, so you at least have that in case you need the lineart for future panels and such.
Here is the webtoon I'm talking about btw: