Well, I moved it the Community section, which is the best for this topic.
Well, in my experience it depends very much on the time you spend every day learning and using it. It's the same than it is with everything you want to master. I used to talk lithuanian everyday as much as a I could. It took me myself maybe over half a year to be able to maintain a regular text conversation on the internet, using declension tables and a dictionary. I think it took me around two years to do the same without help, maybe a little less than that. But of course, always simple conversations, on known topics. Nothing way too complicated. In three years I knew almost every grammatical aspect of the language by heart. What I lacked the most was word colocation. I wasn't sure what verb to use to express something related to certain noun, which I knew was not supposed to be translated word by word.
So on my third year I arrived to Vilnius and took a two semester Lithuanian course at Vilnius University. The first they we had a level test. As it was multiple choice I had almost no problem completing it (except for "berniuko buta___ gero" I think) and ended up on the highest class. After that year I graspped every concept regarding verb aspect, and so on that is simply explained but dificult to unconsciously feel.
My vocabulary is still improving (or worsening) and will always be. But I can say it took me at least four years.
But what do I know? I'm just an Argentinian.