Language

why not do both at the same time?

Both are very different languages even though they're close. Personally I'd start with Japanese since I find it a lot easier to learn. But since that is my own personal opinion, you may want to experience a bit with both and start with the one you find easiest, as both languages do take time to learn and master (▰˘◡˘▰)

As for how long, it all depends on your own abilities. You may pick it up quick, at a constant pace, or slow. Either way as long as you have that motivation and study everyday you should be able to learn it at your pace ٩(๑❛ᴗ❛๑)۶

Korean is more easy to learn than Japanese imo.
The Korean alphabet(Hangul) is also a lot more easy than the Japanese alphabet.

I learned hangul all by myself when I was only 13-14 with Korean sites and a Korean language book.
I heard both languages are similar so try to learn the language that seems more easy or useful to you?

I can read Korean very fast and I'm good at understanding a lot of Korean bc of Kpop that I discovered in 2009.
I watched a lot of Korean movies-dramas with subs , read korean webtoons(in Korean and with the drawing it helps to understand and learn new words), watched korean vids on yt, sites that teach about the Korean language....

There are a lot of videos and sites online that teach you Korean, there are also penpal app like Hellotalk where you can find Japanese or Korean people who can help you learn their language

idk about korean but if you're going to learn Japanese, get ready yourself to learn three kinds of characters
Hiragana, Katakana and Kanji

Hiragana and Katakana are easy, Kanji is chinese characters in "Japanese-way" xD

I am good at learning new languages, but Japanese stymied me ... I took it for a year at University ... and only know a few phrases and such ...

Korean it's easier to learn

I find that it's very helpful to regularly talk with people in the language you're learning. Ideally, these people would be fluent, but it helps even if it's with a person who is also just learning. Some people even keep journals or diaries and write daily entries in the language. I think it's a good idea, but I haven't gotten to it because I'm horrible at writing entries on a regular basis.
Good luck! :D

i think it's better to learn japan first, like dud, people appreciate japanese more in indonesia than korea (i think?). it's of course up to you, but i don't recommend learning both at the same time.

Japanese has easy pronunciation but a bit difficult to learn (hiragana, katakana and kaniji) when compared to Korean.

I think you should consider which language are you attracted the most? I want to learn both, but decided to study Japanese because I want to read and converse in Japanese. I enjoy Korean shows but that's it. Meanwhile, I want to fully understand the context in Japanese and being able to express myself in that language.

THANK YOU guys for the advices! I have decided to learn korean first since i like to read and probably will go there too. Hope it will go smoothlyヾ(❀╹◡╹)ﾉ~

korean is waaay easier to learn, and it can actually help you speed up the learning process with japanese! I recommend korean first