Can I get a decent score on the SAT Japanese subject test just through this course?
Will the duolingo course be enough or will I need to prepare something extra from other sources? p.s. Before starting the course I already had knowledge of hiragana, katakana, some vocabulary and few kanji with basic grammar. So now I have an advantage of being able to understand and not get easily overwhelmed. :D
Thanks in advance!
Depends what you mean by "decent" but it will be difficult because many native speakers (especially of Asian languages) take the SAT subject test for their language. If you miss a few problems you will already have a garbage score. I would not take SAT language unless you are a native or very fluent.
Your chances with Japanese are quite a bit better than Chinese or Korean, but it will still be difficult. See here for percentile breakdowns from College Board: https://secure-media.collegeboard.org/digitalServices/pdf/sat/sat-percentile-ranks-subject-tests-2016.pdf
You can see that for non-language subjects, a 750 is a pretty good score. In general only 10% of people get 750 or above and 90% get below. However, for languages people get MUCH higher scores because natives take it and miss very few problems. For Chinese 78% get 750 or above, for Korean 81% get 750 or above, and for Japanese 52% get 750 or above.
So clearly Japanese is easier for non-natives to compete in than Chinese or Korean but it's still much harder than non-language subject tests. If you want to go for Japanese it is doubtful Duolingo alone will bring you anywhere near proficient enough to compete with native speakers.
Don't mean to discourage you, but just giving you fair warning.
This is all true and good and useful information. Colleges know perfectly well, though, that lots of native speakers take SAT language tests, and they read people's scores accordingly. It's true that you aren't likely to be in an especially high percentile unless your Japanese is very strong, but colleges know how non-native speakers with various SAT II scores tend to perform in their foreign language classes once they matriculate, and plenty of the people taking those college Japanese courses aren't native speakers.
Of course, just how good your score needs to be to help your application depends hugely on which colleges and majors you're applying to. This is exactly the kind of thing that phoning admissions offices can be really helpful for. They won't tell you what score you "need" to get in (since admission is based on lots of different factors), but they are usually pretty open about giving the median and/or mean scores of people who've been admitted over the last few years.
Taking the SAT Japanese test might or might not make sense in your particular situation, but it's worth investigating what kinds of scores people in the programs you're looking at tend to have, rather than just being automatically put off. Colleges are aware that lots of native speakers take language tests and they have practice taking that fact into account!