Well...to illustrate, here's the evolution of my WIP's plot:
2005-2006, in Iraq the first time -- "Wouldn't it be cool if there was a school, like for soldiers, and there were four different...oh, houses or something...and each taught a different skill, but they were all used together in war?"
2006-2007, home from Iraq -- "Hmm...what if each of the houses was a different type of combat...and it'll be like final fantasy. I'll have a knight, and a mage, and a witch who can talk to animals, and a healer. And they'll all be girls, and they'll save the world. Awesome."
2007-2008, in Iraq the 2nd time -- "Hmm. This school thing isn't working out so well, and it's coming off WAY too much like Harry Potter. Let's drop that angle, and add a princess from a foreign land coming in, and SHE'S the real hero, the other four just protect her. And the bad guy wants to destroy the world."
2009, current -- "Dude there is WAY too much twee-ness running around in this book. No more SCHOOL, raise all the ages, cut the ridiculous melodramatic speech in Act 3, make the princess a bit of a badass, and give that bad guy some motivation. He's way too much of a cardboard cutout."
So...lessee...4 years and counting? But to be fair, I've really only been seriously writing this story for a year and a half.