[kirakira]

Depends on the person and also depends whether you know how to speak Korean or have a Kanji backgorund or not. If you say no to both of the above characteristics, you will probably be able to express 50% what you want to say using short sentences but no where near fluent even if you stay in Japan for 1 year.

If you can speak Korean, you can probably construct more complicated sentences and also speak much more fluently in just 1 year.

If you have a Kanji background, you will have a boat load of vocabulary but grammar wise you will still struggle.