There are a lot of different answers here for you to choose from.
1 – Children’s Picture Book 3-5 months pending what needs to be changed upon submission
2 – Fiction from 6- 12 months
3 – Non-Fiction 3-12 months
Children’s picture books are easy. Basically the issues are with making sure the images are the property of the submitting artist. Generally the writer is not the artist, thus a lot of work. When the writer is also the artist there is less work needed. Editing of the text takes little time but the compiling and formatting can be a nightmare for not all pictures are the same size. Remember, they need to be at least 300 DPI in order not to be fuzzy.
Fiction, regardless of the genre, can take 6 – 12 months pending on the rewrites needed to clear up the narrative. I requested one author to change the tense in the work in order to publish. They returned it within 3 weeks so we moved faster on that one. Again, checking for originality is important for publishers, so this could delay the publication as well.
Non-Fiction either is short or long pending on fact checking. If it is a memoir, then it will not take as long for that is subjective. I remember telling Melissa Grunow (Realizing River City, published through Tumbleweed Books) that she had to remove 50% of the “I”s in the work. Thought she was going to stop writing. When she completed it (2 months later, good girl) the result stood out tremendously. Her response was, “I can hardly believe the difference that made in reading the work!” There were other issues that we tackled with her and finally finished it 7 months later.
There is no real time line, it takes as long as it takes. If you feel your publisher is dragging their feet, you can always ask why it is taking so long.