A few weeks ago, a client asked the question: how long to hear back from a publisher?
Me: Not at all. The rejections come in first. When an editor knows it’s not a fit for them, they respond quickly. When they are interested, they bring the proposal to an acquisitions board meeting where the board examines the proposal and decides. The acquisitions editor whom your agent contacted will make a case for the book and lead the group through highlights of the proposal. They may suggest changes so it fits with their audience/vision/etc. or have questions for the author. So there can be some back and forth.
So, if you haven’t heard yet: