This question has filled my mind for a few days. I can spend 6 months on each language, work hard and get fluency in it. if I wanna study 6 languages at the same time, does it take also 3 years like when I study each language in 6 months?
The OP can be interpreted in many ways, and it's not specific enough, so I'll extract what I think the real question was supposed to be:
You want to learn 6 languages from scratch. In which scenario will you make the most progress, assuming you'd spend the same amount of time in 1) as you would in 2)?
1) Learn them one after the other, spending 6 months on each
2) Learn them all at the same time for 3 years
There have been many questionable statements on this thread so I'll quickly comment about some of them. OP - the word "fluency" can mean anything, so if you want people to know what you mean please use the CEFR scale. Unless you have a lot of time on your hands, 6 months will not get you to a high enough level to make this an intelligent plan, because you will have to spend too much time maintaining them afterwards. I disagree with "Myself" - you should reach C1/C2 before moving onto another language, not just intermediate. I don't recommend the method Katya mentioned because it's still not enough time per week (5hrs/lang) to make satisfying progress especially in the difficult languages. OP - language isn't like physics or math. Beginning 2 or more languages at the same time is a bad idea; most experienced learners stagger them. Pure "listening and speaking" methods yield very poor results. You are not a baby, and don't learn like one.
Regarding to my personal experience, it's better to focus on one language untill you achieve at least an intermediate or even upper intermediate level in it. Then you can start learning another language and improve the first one besides.
But I started some of them before! only Chinese and Turkish are new in my list.
actually I believe everyone has different potentials, so it differs from me to another person. Some people's comments on this discussion let me down! But I'm back again! I will make it with hard work
I haven't tried it myself, but I have seen other people do it. I knew a girl who studied Japanese, Chinese, and German at the same time and did quite well in all of them. I also knew Slavic majors at my university who were learning more than one language, in addition to Russian, at the same time. Learning a language depends on the amount of work you are willing to do. If you can commit yourself to the effort it takes to do multiple languages, go for it.