I'm not sure what you mean about a "grasp of understanding," but it's going to be hard to make significant progress in six to eight weeks. Japanese is a difficult language, and it takes some people years before they are able to communicate fluently.

That said, you can get a good start in a few months' time if you are able to find a good teacher. Both private teachers or local college classes can be great if you can find one in your area.

I really think you should have a teacher in the beginning, especially if your goal is to speak Japanese. If you are only interested in reading and writing, you may be able to get by without an instructor, but there are too many potential hangups that you will miss in speaking Japanese by self study.

Regards,
Kaeru