
I finished the novel. “I wonder how long he’ll think that” is bit confusing. There is a whole back story you don’t know yet that reveals the whole plot of the story. Wooyeon thinks Dohyun fakes a smile with him but Dohyun says later In the novel he never once faked a smile with him. So it might be him saying he likes Wooyeon and his smile at Wooyeon is real because…he has feelings.

Just read that part, and what he means is,
" I wonder how long it will take Wooyeon to think that I genuinely like him, and I'm not pretending to be sincere"
Coz in the chapter they kiss, Dohyun says if you were going to listen, I wish you had listened to the whole thing and if Wooyeon had listened, he would have heard Dohyun say that he genuinely likes him.
So the " I wonder how long he'll think that" is a prelude to Dohyun revealing that he genuinely likes Wooyeon
Who's reading the novel? Someone explain to me what he said at the end of chapter 43!! He sounded like a jerk.