Time needed to get good at any language is a largely individual thing. It's my 7th year of learning it, with varying degrees of intensity, and I feel that I know less than what I don't know.
I'll start with good news. As a Pole you're covered in all but one sound that isn't directly present in Polish but won't be difficult to learn anyway - nasal 'n' ん. Only challenge in pronunciation is intonation and remembering about long vowels. I've met with an opinion, that the grammar is easy. Granted constructions aren't very complex but there are many of them. That is bearable but this grammatical simplicity can be a pitfall because all the specifics you're given in Polish are in most cases simply not there in Japanese.
So in most cases grammar won't come to your rescue when you have problems with understanding what was said, you need to rely on context. And here it gets crazy because of the terrifying amount of homonyms that Japanese is burdened with. Sometimes I find more than 25 entries for a two syllable word written phonetically. Even if you know a shitton of vocabulary you still need time to compute what is being said and select appropriate words, which might become an issue because Japanese can be spoken quite rapidly because of it's syllabic nature.
Maybe you're thinking that you'd start with being able to understand written form and then challenge the speech. Think again. When I started learning I kept saying to myself "it can't be that hard, every goddamn fisherman can read nowadays". Even though my mind kinda broke after about memorizing 700 of characters in unhealthily short a period of time, I held that idea until I've seen natives making mistakes. There is an official set of characters that won't have pronunciations included in publications they appear. It's over 3000 now (yes, it grows). Largest character dictionary has about 45000 entries but a large chunk of that is only used for names of some objects in the middle of nowhere or whatever. Let's say 5000 would be a reasonable value to be considered somewhat eloquent.
Knowing a character entails being able to write it, recognize it, knowing it's readings - pronunciations, understanding it's meanings and knowing what words it forms with other characters. You kinda can cut corners like focus on the passive knowledge of writing (because most of the time you'll end up writing through some sort of computer input system anyway), sometimes guess readings out of character compounds, figure meanings out of context and so on but there's still a fuckton of vocabulary to learn. Learning process is made additionally difficult because searching a word written in chinese characters that you can't copypaste is a chore when you don't know the readings, even with electronic dictionaries that you can draw the characters on with a stylus.
You've perhaps heard that Japanese are very polite. Japanese language possesses an entire systematic category of politeness with a complexity that makes natives themselves struggle with it, encompassing humble language & respectful language and sometimes expressions that are supposed to be humble but can only be said by someone in a position higher than you (like 小生) so you can go fuck a duck because you won't figure that bullshitery out any time soon.
That being said I'm not purposefully trying to discourage you but I can't imagine learning a language just for the shits and giggles because you need to use a language to learn it proper. So unless you want to explore some aspect of Japan I'd say don't bother or try learning it for a year just to get an rough idea of how different a system of human communication can be.