That depends on your definition of fluent. Some people define it as being able to speak "flowingly", which is to say, being able to effortlessly say a sentence, but not necessarily speaking the language perfectly. Other people define it as being able to speak indistinguishably from a native speaker.

see 'how long should it take to be fluent in Mandarin'

it takes more than 2 years,but it is up to your concentration and interest.if you are keeping more interest then you gonna get it in 6 months,but if you are still weak than GOD knows.and i think if you gether with chinese people more ,the more you will learn.

I've seen video of a 5 yr old boy from the states who lived in China for about 9 months and was fluent in Mandarin. 
I know adults who've lived there for years and say they can't speak a word.

Some people learn languages easier than others, just like some are better at math and science, some in literary fields. For some people it might come fast, and some, not at all. Learning a language isn't a race... if you are truly invested in it, then you'll learn.. at your own pace.

Sorry,i was too young to remember.Maybe it cost me one year or two,i guess.

The time, that it takes for someone to learn a language, depends highly on the individual. I would say it takes about a year of immersion to speak the language efficiently for most. However, depending on your practice and what type of environment you are living in, culturally immersive or not, this time could be expanded or slightened. It takes many more years to speak on a native level in a non immersed atmosphere than it does for a fully immersive one. In other words, living in China, you will pick up the language far quicker than living in the States or in EU. Speaking practically however could take you as little as 3 or 4 months. That is simply speaking the basics and relaying essentials. I hope I've helped.