Several people have emailed me lately asking when they should give up on their books. One even asked if there was a certain number of drafts or rejection letters that you had to hit in order to give up.
The honest answer? You don't have to ever give up.
Seriously. You could spend the rest of your life revising and rewriting the same idea, querying every new agent out there, requerying the ones who expressed interest in previous incarnations, etc. You don't ever have to stop writing that one book that's in your heart.
But if you're a writer, there are going to be more shiny ideas. Sometimes they'll rise up from the ashes of another book like finding sea glass on the beach. Sometimes you'll be able to scrap the idea you have and go with this new idea, using bits and pieces of the old stuff to build the bones of the new.
Sometimes all you need is a break. Drawer the book for a few months, a year, however long it takes for you to work on another project. Maybe you'll go on to land an agent and a book deal for that new project. Maybe your publisher will even take that old book as the option in a two-book deal.
Sometimes books just aren't saleable. Sometimes they are too different, or too similar to other things out there and publishers just can't take the chance on you. You have the option of waiting a few years to see if the trends come back around again, or self-publishing it. If you've got great CP's and a small budget for editing, you can put your own book out for very little cost. Use Kickstarter to find that niche of readers willing to back you. Prove those non-risk-takers wrong.
The point is, it's up to you when you want to stop working on a book. I can't tell you that twelve revisions means it's time to give up. Getting that 100th rejection letter isn't a death sentence.
If Howard Schultz gave up after being turned down by banks 242 times, there would be no Starbucks.
If Walt Disney had quit after his theme park concept was shot down 302 times, there would be no Disneyland.
If JK Rowling stopped after being turned down by multiple publishers for YEARS, there would be no Harry Potter.
Edison allegedly said, "I have not failed 1,000 times. I have successfully discovered 1,000 ways to NOT make a light bulb." And eventually he figured it out.
And it's great to remember that even if you quit one book, it's not a forever sentence. Years from now you are more than welcome to pick it up again and it will open it's arms for you like you never went away.