The Stages of Publishing and Time:

1. The author writes a book. This may take anywhere from a month to many years.
2. Agent sends book to editors who might be interested in it. This may take anywhere from a couple of weeks to years.
3. Editors make offers on the book. Can happen in a day. Can take years. May never happen. I have many books that have never made it past this stage. For whatever reason, no offer was ever made.
4. Contract negotations occur. If you have a good agent, this will take a couple of weeks at least, and can drag out for several months.
5. Editorial letters are written and sent to the author. Author has a chance to decide whether to agree with editor about making changes or to refuse to make changes or to make other changes the editor has not necessarily suggested. This usually takes about a year, but can go more quickly. It can also end in a book that never comes out because agreement is never come to between editor/publisher and author.
6. Book covers take months to create, even if they are done with photography. The publisher's art director will have input, as will the editor, and other people from the publisher. The author is sometimes given a chance to say something about the cover (though not always). This will often happen in tandem with the editorial process, but not always. Problems with cover art can occur, and then the art department has to start all over again. Takes months.
7. Often ARCs are created at this stage, when an editor feels confident enough about the manuscript to think it is ready to be sent to reviewers. ARC (Advanced Reader Copy) are paperback, cheaper versions of the book, sometimes even without the final cover on them. They are free (even though you sometimes see them for sale on ebay) and they wlll have errors in them.
8. Manuscript goes to copyediting. Usually takes a couple of months for copy editor to go through manuscript with fine-toothed comb. Authors often are given a deadline at this point to get manuscript back to publisher. Sometimes the author has only a week to make final changes, sometimes several weeks.
9. Copyedited manuscript is turned into a "galley" which is typeset the way the final book will look. Authors often get a chance to make final changes, though contract will limit exactly how many changes are allowed. This isn't the time for major alterations. Takes a couple of months.
10. ARCs are sent out to major reviewers and publicity for the book begins. Reviewers needs at least a few weeks to read the books and write reviews, though they will usually get a couple of months. Also, ad copy for ads needs to be written, and ad campaigns have to be managed. (Some books have virtually no publicity, but for others, it may feel like the book is being talked about long before it will be available in stores. This is to make people aware of the book so they rush out to buy it soon after it arrives, so bookstores will bring in even more books to sell in subsequent weeks.)
11. Books are printed, often in China because it's cheaper, and the slow boat wait begins. Can take months.

Hopefully, this explains why it takes about two years for a book to move from an acquired manuscript to a finished product, and why it may take much longer than that.