Korean - English
How hard is it to learn Korean from English? Also how long would it take to be able to use it fluently?
Very difficult according to the US Government. Among the most difficult languages to learn. However, easier than per say, Japanese I'd say
That is a pretty hard question to answer.
If you go for 25 hours of class per week and 3 hours of daily independent study (basically, what you'd get if you enrolled in an intensive language course at a Korean university), it would take you 88 weeks.
Not many can put in that much time. Also, every learner is different, so just take this as a really rough ball park figure.
You can factor in your personal study schedule to estimate how much time it would take.
Spending hours on Memrise or Duolingo is probably a good start, but it won't replace the more active study methods that will be required to get past the beginner level.
For more information, I once wrote a post about my plan of becoming fluent in Korean. Check out the reddit link in there for more specific advice I got from fellow learners.
Note that reaching a high score in standardized tests (i.e. TOPIK) does not necessarily correlate with daily fluency in spoken Korean language. Think carefully about what your actual goal is ;)