I am frequently accosted by writers, not necessarily my clients, who demand to know how long it takes to hear back from editors. They have an agent who has submitted a proposal or a manuscript, and they are biting their nails waiting to hear back. I understand how hard it is to be patient; I am as neurotic as the next writer. What they want me to tell them is “six days.” Or 43, or 19, or even 176, as long as I promise they will hear back by then. It is the uncertainty they object to, not the elapsed time.
Which is why no one ever wants to hear my answer, which is: “It depends.” Asking, “How long does it take to hear from an editor?” is like asking, “How long is a piece of string?”
I can tell you that no comes faster than yes, so be careful what you wish for. I can tell you that editors with more seniority generally respond faster because they have to do less hoop-jumping to make an offer and are more likely to know if something is going to get past the acquisitions committee. By the same token, some agents and agencies have more clout, and editors will respond more quickly to submissions from them. Projects that have huge commercial appeal will go faster than projects of less commercial appeal or with non-celebrity writers.
Other than that, it depends on how strongly the editor feels about the project, how much buy-in she needs to get from others to make it fly, how full her list is (not to mention her workload), who’s on vacation, and even how well another book on a similar topic is doing. I’m currently testing an hypothesis about the fullness of the moon.
You may get an offer in a few days or weeks (I’ve had that happen). You may get an offer after many months (Ditto). You may never get an offer. You may never even hear back.
So instead of worrying about “when,” accept that it will be seven months from now, be pleasantly surprised when it isn’t, and get to work on your next project.