It takes about 2 years to get reasonably conversational.

Japanese writing is a mixture of 3 (4 if you include "romaji," English characters) writing systems: kanji (traditional Chinese characters), hiragana (a simply script originally made for women) and katakana (used for words of foreign origin).

One sentence could include 3 or 4 different writing systems. Example:

私の名前はABCのジョンです.
"My name is John (from the company ABC)."

私 の 名前 は ABC の ジョン です.
kanji/hiragana/kanji/hiragana/romaji/hiragana/katakana/hiragana

Think that's easy?

Chinese is way, way easier.

And in Japanese kanji can be read several ways, depending on what it's paired with.

"Well enough to go to post graduate school"? 5+ years, probably.

I think so. About 2 years... This language is not easy to learn if you do not study hard. But I think nothing is impossible. I have learned Japanese for 3 months...Try our best! ...

i have stayed in japan 6years.
i can speak japanese.
japanese language is comprised by various country words， of course they have their own characters,which is called "hiragana".
"kanakana"or"romaji" is come from western countries(english mainly),the pronounciation copied local pronounciation, but the characters is wrriten in japanese way. which characters is called "hiragana"or "romaji".
you said that many characters are the same as Chinese,because they learned words from china in tang dynasty,and those words maintained, some characters pronaciations are similar to chinese , some are not.

i hope that what i said will be helpful.
good luck

Quite a long time to learn Japanese well, you know. Try to continue learning. I've just passed N1 this year.