hiya carol!! welcome to the forums.
i cant speak on the webtoons issue as i dont use the site, but as for this...
i study art full time (with mondays and wednesdays off) and its a fairly intensive course, on top of which i do Various Other Things. i set aside saturday to monday to work on my comic - usually a batch takes me two days, lately its been taking three (new program, more intensive method). when it takes two days, it means i have leeway to sacrifice saturday for a Various Other Thing, and work into monday - otherwise, monday is touch ups and homework.
the most important thing to keeping on top of schedule, especially when balancing comics and school, is having a nice big buffer. i have about 22 pages worth of buffer, and thats my baseline really - i want to get it up to thirty - which last for 11 weeks of updates. the pages i draw each weekend wont come out for 11 weeks. that means if i need to sacrifice a weekend or two to catch up on smth else, i totally can, and its fine so long as i catch up later
and this is the second Big Trick to keeping on top of schedule. work in batches. you might be able to do one page - sketch to colour to letters - in a day, right? you could probably do five pages, complete, in two days. then, if you release two pages in a week, youll always be building on your buffer, always creating a surplus to what youre putting out. you could skip a week and it wouldnt make a jot of difference.
EDIT: for clarity, working in batches would be where you sketch five pages, and then ink all five, and then colour all five, etc - rather than doing them all one by one
theres a lot to be said for working fast. working in batches is a big part of it, but look for other ways you can cut corners - special brushes can speed up the shading process, things like gradient mapping can speed up colour, and having an art style that works with the fill tool speeds up everything. so much.