I have been on submission since the beginning of January.
The first response (a request for a pre-contractual revision) came exactly one month later. Other responses trickled in over the next couple of months, including some editors who had similar concerns about the book as the first editor and were willing to see a revision.
Towards the end of May, my agent and I decided that I should do the revision, and we should do a second round of submissions, sending the revised book to the editors who had expressed interest in seeing it again, the editors who had not yet responded to the first version, and a few additional editors.
We started that second round of submissions in mid June, so about six weeks ago. No responses yet, though my agent says that things can be slow in the summer, and that it's easy to say no and takes longer to say yes, so no news can be good news.
I was one of the (rare) people who found an agent extremely quickly, so I wasn't really prepared for the drawn-out nature of this part of the process! I'm just trying to not think about it. And I've written 63,000 words of my second book in the meantime, so I've been using my time well.
I'm sure people will come on here who got a contract in a week, and others who waited a year. It really does vary very widely.
My agent and I did discuss timing when it came to submissions and in general, she said, summer is slow, and the end of the year can be bad (because editors may not want to take on any more new books). When is the best time to submit will be part of discussions with your agent when you are ready.
Oh, and I'm remembering that my agent did say that things seem very slow this year -- that publishers are taking a lot longer to make decisions than they have in the past. That's a factor too. It makes me wonder why, after years of fiddling around with different fiction projects I had to decide to actually finish one when the global economy was melting down and the publishing industry was imploding....
