While there are many influences, such as knowing another language in the same family, that are individual to each situation and person, the common notion among people I've spoken to regarding the matter estimate approximately three years. How have you found your own progress with learning a new language? At what point have you considered yourself to be fluent? Have you lost your motivation when attempting to learn a language and given up? Or how do you keep yourself motivated?
In my own situation, I relocated from England to Greece and have been immersed in the language and culture for the past 18 months. Despite my own wishes and plenty of practice (though, admittedly not quite enough), I'm not able to communicate as well as I would like. I can get by with my day-to-day requirements, watch the news, read the paper, order my coffee (oh so important, that last one) but I am nowhere near the level I want to be. While it is frustrating, I try to remember not to be too hard on myself, as it is a difficult language that is far removed from the languages I am fluent in (English, German and Italian) and which were all learnt when I was a child, whereas I'm now in my mid-thirties and it is a totally different experience. Having lived in and travelled to a number of countries I used to consider myself quite skilled with languages, but I have had to rethink that after the struggles I'm experiencing trying to learn (Modern) Greek. I will persevere, simply because to me, there is no choice to be made, I both want and need to learn the language if I wish to continue living here, which I do.