Time - how long from idea to publication

I read a review of my book the other day that found it "strangely similar" to another book that came out six months before mine.  I have another friend who got a similar review -- comparing her book to another one that came out six months before hers (and in that one flat out accusing her of writing a "knock-off").  And another friend who's asked if she wrote about unicorns in her books to escape the current trend of "vampire, faeries, and zombies."

This got me thinking -- being surrounded by wr