Last month, I completed the Welsh tree on DuoLingo. By "completed" I mean that I got all of the units up to at least level 2. That means I've covered material up to and including Canolradd level. I should be practically fluent by now, right?
Errr, no. I'm doing OK reading through my Welsh short stories book ("Ffenestri", by Lois Arnold - I highly recommend it) and I'm understanding more on Radio Cymru each time, but I just don't get enough chances to speak for my brain to engage quickly enough when I need to put a sentence together.
That said, DuoLingo has definitely helped broaden my vocabulary, and I'm often surprised by the sorts of sentences I can put together, but I don't think DuoLingo alone can get you fluency. For that, much more exposure to the spoken language and the way people speak it is needed.