Duolingo is a great kickstarter from zero knowledge, in my experience it also leads to very fast reading proficiency.
I'm convinced I can make sense of a newspaper within weeks and read novels within months in any language just using Duolingo (maybe longer if I also have to learn a different writing system). I did it with Danish years ago. I could read newspapers after a month and after four months I read a few novels at a slow but still enjoyable pace.
Or another example: my daughter was struggling with English at school and I bribed her to keep a 200-day Duolingo streak. That was enough to allow her to understand sources in English about her hobbies and interests, and now after a couple years her level is way above what is taught at school, she aces every test effortlessly.
Fluency... English is not my first language, but I can write, read and understand it pretty well. I even "think in English" at times, but I have never spent more than 4 consecutive days in an English-speaking country, and my fluency suffers a lot comparatively.
I can speak about anything, even give a talk or teach, but my accent is very thick, I'll stammer and stutter and stop here and there to find the right turn of phrase, or struggle to pronounce words that I have written a thousand times but never said aloud.
So in my opinion if you want to speak well, you have to speak a lot and I don´t think Duolingo is very good for that.