Seems like it's 35 episodes for me... The only 50+ episodes drama I finished was New Tales of Gisaeng, and that's one of my favorite dramas.
From time to time, I start watching 50-episode dramas. However, I see myself getting tired around the 20th episode. I put Rascal sons and Ojakgyo brothers on hold at e21. And I'm struggling with You're the best Lee Soon Shin right now xD