[Tsuwabuki]

It depends largely on what you mean by "it." When I arrived in Japan, I had a very cursory understanding of enough grammatical elements that as long as I had vocabulary, I could communicate. I found, however, I had spent so much time on grammatical structure that I didn't know enough words to actually plug in. I would say after no more than two or three months I knew vocabulary for most of my daily tasks.

I can now have a rather rambling conversation in Japanese for hours with no real issue. Sometimes I need to ask for the other person to slow down or give synonyms or examples of words I don't yet know, but it rarely causes the conversation to stop. I even understand about 80% or 90% of complex legal transactions. Just last night I played translator between a new arrival and SoftBank (cell phone provider), he wanted an iPhone like I have. Each time a term of the contract was explained to me, I had to turn around and explain it to him.

So for me, I would say "it" being "functional" or even more than functional would be, er, maybe six months of constant, daily exposure. Since I only really studied grammar before moving to Japan, I have nothing to offer in the way of studying Japanese while not using it, at all times, every day.

Reading wise, I would say has been much longer. I only recently became comfortable enough with my acquisition of kanji to start reading popular manga. Light novels are probably going to be my next step, assuming they have furagana. Right now, I know about 240 kanji (elementary school first and second grade kanji) completely with on yomi, kuni yomi and ei (english) yomi, as well as how to properly write them. There are probably at least fifty or so random other kanji I can recognise in some way, but cannot write yet. I've been studying kanji for four hours every week day between classes, but I do feel my acquisition is not as quick as I would like.