Well, if it's about Duolingo, I don't know. I use it for revising. I wrote an annoyingly long post, but I included a too long;didn't read (tldr) at the end!
Mostly in 2011 I started learning some Italian in February, in prevision for a one month Mediterranean trip with school. I studied grammar, listened to songs and wrote them down and tried to translate them. I also printed out stories for kids and sat in the library with a dictionary, making sense of it as much as possible, not really *too* concerned with getting -everything-, just the general meaning and some extra vocabulary. As I learned more grammar and words, I then went back to these stories and songs and was happy to see I could understand more and more! I leafed through language learning books, downloaded a few podcasts on my iPod, actually copied down grammar pages and expressions books (well, I mostly did this when I had to watch over exams and stuff like this). I had college though, so it was not full time, but it was a rather regular thing since I was very curious and really enjoyed the puzzle.
But I should mention my native language is French; it really made the whole thing easier. And when in May (or June?) I went to Italy, I could ask my way around and explain what I needed when I went to the bookstore (in this case, an Italian edition of The Little Prince). When I read through the Little Prince, I was happy to see I could make sense of it, even though I could not really pretend I could read it all or read it well.
When I got back home, the motivation of a trip to Italy being gone, I kind of stopped learning it. Until I went to university and they had semesters abroad in Rome. I applied to that and was told I needed to have done Italian classes at uni and at least reached Intermediate 2 (it goes beginner 1-2, intermediate 1-2, expert 1-2). I did not want to have to do 3 classes before applying, so I studied in a frenzy and asked the Italian teacher for a test. She received me and gave me different written tests and finally asked me to speak with her a bit. I did good with the exercises and written part, and stumbled over most of my words in the oral test (not too surprising seeing I mostly taught myself and practiced alone). She mentioned that my spoken Italian was not too good, but that otherwise, my results were good enough to put me in Intermediate 2. So I joined her class and gave it my best.
Before I went to Rome, I actually also did Expert 1, and then I flew to Europe, spent a month or so in Germany before getting to Italy. The courses that were in Italian were really challenging. The Italian Literature one was horrible -- the teacher was the very stereotype of a passionate Italian speaker, speaking fast, going off many tangents, etc., etc. I had no idea how to take efficient notes (in French? in Italian? I had already lost track of what was being said by the time I jotted down a few words). And the stuff she had us read was HARD. Giovani Pascoli: some old poet from a century ago who really liked dead languages, old Italian dialect words and very field-specific words. I could understand a word out of three in every of his poems, and before that, I thought I could do good in Italian!
After this semester of hell, I did not use much Italian. I got out of practice and stopped studying. But it was enough so that on my trip to Milan, I could have a nice chat with the tea lady from the café; that at the Expo, I could read the brochures and get it, and understand the speeches and explanations given at various expositions; that I can now watch Pokemon in Italian on Netflix and, when people post articles in Italian or when there are threads in Reddit on /r/italy, I can actually read it with ease, checking a few words out in need. I can also write in Italian (not too formal). I don't think I'm fluent though -- when in class we had to debate in Italian, I felt so much frustration when I heard arguments given on subjects I cared about and realized I could not really give a decent response with the words I knew. Like -- I could say my general point. But I could not convey all what I wanted. And that is a very frustrating feeling, to fall short and to feel stumped.
tl;dr:
3/4 months autodidact learning with all resources I could find + short trip to italy + a few intense weeks of autodidact revision + 1 semester Intermediate 2 university class + 1 semester Expert 1 university class + 4 months study abroad in Rome trip + random and spaced practice moments and opportunities since
And I think it depends on the people. 