Honestly, whatever is comfortable with your schedule and won't run you ragged trying to maintain 
Because the next most important thing behind frequency is consistency. It won't do very well if you post once a day for a whole week, for example, but then fall off the face of the earth for a month or more afterwards.
Like, ideally you would post once per day as that's the maximum frequency your series will appear on "fresh" (on tapas at least, not sure about webtoons. Here if you post twice in a day your series won't get bumped up the 2nd time).
BUT that's not feasible for most people. Once a week is pretty common for hobbyist comic artists who have other things going on. Bi-weekly isn't super uncommon either. Some folks opt to do like once a month but with longer updates, and most people who do multiple times in a single week do really short episodes, such as if you're doing a comedy gag-a-day type comic.
It's fine to break your schedule sometimes (or change it if you find that it's too much or too little) but generally speaking you want to be consistent more often than not for consistent series growth.
