If the episode has like, explicit nudity, sexual content, and gore. I'm pretty lenient with what is considered mature or not. If there's anything triggering (talks/depictions of suicide, abuse, etc., dysphoria, homophobia...) I'd usually add a warning before the episode starts.
Honestly, my criteria for tagging a chapter as mature is one of the main reasons why I'll probably never do so - not only because I post on Webtoons as well - where you can't tag individual episodes as mature and can only tag the entire work as so, and where the rules on the mature rating is stricter compared to tapas - my comics never go to that point, and it's something I'm not comfortable drawing or writing.