For adults, language learning is done through different pathways in the brain than when you were a small child and you would not learn Japanese by osmosis- i.e. just being in Japan. Even in Japan you would still have to be in a Japanese-only environment as much as possible (not always the case if you are around other foreign students or co-workers) and you also have to put in a lot of effort to practice and study.

In one year, with plenty of effort, you could reach a level where you could have a reasonable conversation with someone. Three years at least for real
fluency. Written Japanese takes even longer to learn. People who study these things have said that it takes 3 times as long for English-speakers to learn Japanese than it does to learn a European language because the grammar and writing systems are so different.

It takes Japanese people years from childhood to adulthood to study and become fluent. So its hard for someone to go from nothing to fluency in 1-2 years.

I know English and Chinese and have found that Japanese progress is much_much quicker than chinese.

Educational material is superior, no 2 types of characters (simplified, traditional) etc. I've studied fore a month and know katakana, hiragana and probably about 1100 of the kanji (not what they sound like mind you). Seems like a lot of loan words from both languages.

My goal is to be able to read basic comic books within a year. Don't know if it will be possible as my mind is old and slow.

Plowing through pimsleur, lets learn japanese videos, and a half dozen books and plan to spend a week every 4 months in japan. I may try to hire a teacher - but I find I learn better if there are at least 1e to 3 other students that I can learn from in the course and I don't want to be slowed down.

My short answer is "a lifetime."

One can learn survival Japanese rather quickly when totally immersed into the culture. Its basically sink or swim...
How motivated are you? I was highly motivated to learn Japanese earlier in life. Especially concerning work, culture, food and communicating with my lovely then girlfriend now Wife.
After a few years, I took a pretty long break because We moved back to the U.S. I have recently began studying again with plans to retire in Japan.
You also need to ask yourself to what degree do you want to understand the oral and written form. Just basics or able to read and write it as fluently as a native. My goal has always been somewhere in the middle of that bell shaped curve.
With regards to age, like another poster said the younger the better...Children pick up languages like a sponge. Your brain keeps improving into early adulthood if I remember right. From then on its downhill baby.
Theres also physical limitations like hearing loss. I personally have a hard time hearing consonants. And with background ambient noise, forget about it. Talk about a minor hitch. I often hear a totally different word or phrase than what the speaker intended. Lip reading helps, but some women mistake watching their lips for breast gazing.

Now, many years have passed and i've decided to learn japanese. My husband wanted to give it a try too, so some 4 months ago we started with the basics we found on the internet.We are both in our fourties and he is quite intelligent. But... I really don't know why, he does not seem to be able to assimilate this new language. I, on the other hand, have progressed pretty fast. When listening japanese music or watching movies or TV, i can distinguish every word clearly (even if i don't always know the exact meaning) but for him it is impossible to hear a precise sound. When we learn new vocabulary, i will not forget it, but he does as soon has he doesn't see it anymore. Now he wants to quit because he is getting frustated.

We have the same background, the same age. He and i are living the same learning experience but are not getting the same results.

'' Japanese in 30 days ''