what ratings classify a drama as a success?

Nowadays it’s the same for all channels. I’d say above 10% is successful, above 15 is a really successful. 

Maybe 1 or 2% less for cable channels. 

Jtbc & OCN definitely have generally less high ratings than the others though.

I also find that it’s different depending on the target audience. Dramas that are more popular with young people only have lower ratings since a lot stream online.

For older dramas around 5-10 years older. It would be above 20 (public), 5 (cable).