I have published five books and been on the editorial board of two more. My most recent book is a textbook. Over the last ten years I have written a memoir and in the last two years have been seeking representation. As a scholar, this is an entirely new world to me even though publishing and writing are not. I feel like I'm working in an alien landscape :-).
Here's what's happened. A friend of mine who is a prof wrote a book and it went popular. A major publishing company bought the rights. As a result, she began working with an editor there. When she heard about my book, she thought it could go popular, too. So, she very generously put me in touch with that editor. I met with her, she read some of my chapters, liked them, gave me suggestions for revision, even edited my query letter and book proposal, and offered to query agents for me. All of these were very kind gestures.
Of those agents contacted, one has stuck with me over the last year as I have revised the manuscript. She has been tremendously helpful with ideas for revisions and making the book marketable to a popular audience. She has not signed with me-- I want to make that clear but she has expressed interest throughout our communications. She's a top-flight agent, too, and represents a lot of people who I totally admire. I'm both thrilled at her interest and, well, completely freaked by how novice I am in this new world.
Early last month I sent her a full, revised manuscript of plus 400 pages in which I tackled all of her suggestions as well as I could. Several weeks have now passed. I have sent her three follow-up emails, mainly just politely asking how she is, what her time frame might be, etc. I've heard nothing. Does this mean she is no longer interested? Or, am I being an idiot? Should I be sending this finished ms and/or query letters elsewhere? Or, should I sit tight and just be patient? I don't want to do anything to compromise this agent's potential support of my work or our ability to work together in the future, of course. But I also don't want to waste my time if she's just sending me a signal I'm too thick to get.
She's had just over a month to read it, and she's a 'top-flight' agent, which means she probably has a lot of authors and a lot of things to read. If she gave you a set time in which she'd read it, that's one thing - but if someone only gave me six to eight weeks to read a 400-page manuscript, I wouldn't have any clients ... and I particularly wouldn't like being emailed three times by the author! Being chased up tends to make us go even slower. We already know that we haven't read the manuscript, and being asked about it makes us feel guilty and then, possibly, not want to read it at all. No one in the publishing industry reads at their desk - they all read in their private time. The manuscripts we read in that time thus take on a bit more weight than if we were reading them at our desks, and it doesn't take much to decide you'd rather read one and not another.
I suggest you now just wait to hear back from her - if you haven't heard by early next year, write a letter (not email) and say thanks for all the support she's shown but as you haven't heard back from her you're going to query other agents. I don't recommend you send queries elsewhere right at the moment, because this agent has given you a lot of hands-on support already - and the start of next year is not that far away (in publishing terms).