From my point of view, you can make it to A1/A2 proficiency in a language with Duolingo alone - after that, it gets quite hard. Teaching grammar and conjugations isn't its forté, without resources outside of Duolingo it will get rather tricky.
That being said, I still use Duolingo for my daily dose of language exposure - but without Anki (for vocabulary), youtube videos (to explain various specific topics) and evening courses (which help a lot by giving a structured approach on what to learn in what order), my learning curve would probably flatten out.