I think the maximum you can stay without some kind of visa is 3 months, so if you're not planning on getting one that would be the longest you can spend there.
If you're planning on visiting the larger cities and touristy areas, I found there was quite a lot of English signage and it's easy enough to get by with very basic Japanese skills and a bit of miming
I found katakana quite useful, as well as some basic kanji and phrases like 'excuse me', 'thankyou', 'where is...?', 'do you have...', ordering food etc. If you're staying for a longish period of time and not working, make sure you have a lot of savings... I found that Japan wasn't as expensive as I'd thought, but it's definitely not as cheap say, China or Thailand.
Hope that helps some!