I'm at 1218 days* with Duolingo for Korean, and no, I'm not sure only using Duolingo will be enough.
My husband and I watch TV in Korean to help with learning, and it's taught me multiple words Duolingo hasn't yet.
I learned English in school as a kid, but it clicked by watching TV all er when I turned 12, so I know TV totally works. Then I started thinking in English, and that's how I think it really stuck.
(*I go through each lesson until I feel comfortable with the words it's teaching, and only do 1 lesson per day, so not that intently though I have a long streak)
