That depends on how you use it.

I use DuoLingo as daily practice, and I add a bunch of other stuff to it as well. I did really well learning Esperanto this way, and have learned a fair amount of Spanish and Korean as well. Generally:

1. Duolingo for a couple weeks, blitzing as many lessons as I can
2. Find lessons elsewhere (YouTube, books, etc), while using Duolingo for 5-10 min/day
3. Read childrens books (look up everything you don’t understand) and watch children’s shows (write down what you don’t understand) in the language, and use that to review grammar and vocab
4. Read the news in the target language, looking up unknown words
5. Watch TV in the target language
6. Finish up the Duolingo course while doing the above

Duolingo by itself won’t get you fluent, but it’ll teach you basic grammar (if you read the grammar notes), vocab, and build a habit of learning with a minimal time commitment. Use it as a sort of stretching routine before more serious study.