Is Fantasie Impromptu easy?
Re: How hard is Fantasy Impromptu? The book "Chopin: A graded practical guide" by Eleanor Bailie rates Fantasie Impromptu as Grade 8+, just below the Very Advanced rating. It is not horrendously difficult once you conquer the polyrhythms. There is a slow "moderato cantabile" middle section which is quite easy.
How long does it take to learn Fantasie Impromptu on piano?
Three months is a long time to be working on just one piece. It will take you much longer to get Fantasie Impromptu at performance standard. Your time and effort is probably better spent learning easier pieces to add to your repertoire.