going to japan to become a bengoshi

how long would it take to get my japanese to a sufficient level were i could do this effectively.

and also how long would it take to be able to speak it like a native?

There is no correct answer to this question.

Language fluency is not like a university degree. There's no "you should be able to do this within this number of years" because every learns differently.
You can go to university, major in Japanese, and in 4 years you'd probably be able to pass the Level 2 or Level 1 JLPT.

But JLPT level 1 doesn't equate to fluency and it's not a very good indicator of someone's speaking skills. It's really just a stepping stone.

But if you want a number, zero to fluent in Japanese in possible in 5 years with intense study and in-country living (homestay, etc.)

don't even waste your time, if you're asking this kind of question you'll never become one here. i'm not even sure there are foreign lawyers who have passed the bar exam in japan.

it's widely considered to be one of or the most difficult exam there is in japan. it has something like a 99% failure rate. that's with japanese people taking it. forget about the fact you'll be taking it as a non native speaker.

Most people can't loose their foreign accent. No matter how long they live in a country. One pretty much has to learn a language as a child to speak it like a native.

On the other hand one can be fluent , even more than some average natives, but it takes many years to learn a big enough vocabulary and to understand and use lots of idiomatic expressions, many of them not taught in schools.

Adam Zaman,

As Winterwolf has suggested, becomeing a lawer of Japanese law itself is extremely difficult even for locals. But on the other hand, I know people who can hardly speak Japanese but practices law in the most expensive area in Tokyo. So depending on your ultimate purpose, there may be other ways to grant it.