Waiting to hear back from an editor or agent about a book can be nerve-wracking. Every writer has their own technique for dealing with wait times. At Harlequin, for queries with synopsis, I found I usually received a response within 4 months, more often around 3 months. I say usually because a couple of years ago, I did have to requery due to my submission being lost.
So what did I do while I waited?
I worked on the next book.
I continued to read books in my targeted line, within the genre, and outside the genre.
I read books on writing and revising.
In the early stages of writing, when I received a response about a query, sometimes the editor included a phrase or two about why the work was rejected. I tried to understand the problems in the book and look at my current work-in-progress to see if I had made the same mistake. Often I had and this might be the reason Harlequin asks that writers not submit more than 1 manuscript to the same line at the same time: no multiple submissions.
I revised. I reworked. I submitted something new.
I think each manuscript I submitted was better than the one before it. Even when I didn't get editorial feedback, I figured each hour spent at the keyboard was an hour closer to my goal: being published.