@heidigfx Are you referring to a book or a graphic novel?
I have only very little experience of working on a graphic novel, and Nathal Hale, here on SVS, has a short course that is incredibly enlightening about the process to tackle a graphic novel.
For books, I would normally work with the following stages:
- General thinking time, character development, rough pagination;
- I normally do character development first and have the AD review the characters and approve them (with or without edits).
- Thumbnails for each page;
- Selection of the thumbnails;
- Sketches for all pages - send to AD (contract milestone);
- When the revisions come in, I work in the edits to the sketches and send them again for final approval;
- Color comps for all pages (normally not defined in the contract, but I send it all the same and have that approved too);
- At some point during this process, I`ll send cover proposals. These are a little more refined than thumbnails but less than the sketches. When the AD picks a cover proposal, I’ll do a proper sketch for that one and send it in for approval as well;
- Normally I will render and finish one page and send it in for approval of the final style. Sometimes this is required by the AD, sometimes it isn’t, but I do it all the same;
- And then you paint away.....this is the largest chunk of work time-wise for me, but it’s also the most relaxing. All design decisions have been made and approved by the AD. All questions have been answered. There are various approaches to help with consistency and speed (Lee White and Will Terry discuss some of them, like making all the backgrounds first, etc...), but I think this is very individual and style-dependent and you can work out your own. What I definitely recommend is to write down your process at the beginning (after the first illustration is done) and save your color palette and any texture/graphic/collage element, etc... that you plan to use in the book in a folder (or, if you work digitally in Adobe, in an Adobe Library). You never know: you may need to set the work aside for a while and when you come back you may not remember all details. Or you may end up doing a sequel
So, I have several pre-defined touchpoints with the AD - I modeled this from my experience working as AD and so far (I’m now on my fifth book) this has served me very well. The bulk of discussions and revisions are done at the sketch and comp stage, and so far I’ve had practically no edit requests on the finals.
If you are shaping the contract, I would set deadlines for character development (and approval), sketches (and approval), cover comps (and choice), cover sketch (and approval) and final cover (normally earlier than the interior) and of course final interior.
Extra to those, you can set your own milestones for color comps and for one sample illustration and have those reviewed as well.
If you are worried about the work dragging, you could set a deadline for the approval or revision requests (within x weeks from delivery) and make it clear that you can only keep your deadline if the AD keeps his/hers.
If you are working with somebody who has no experience of illustration you may want to be extra careful and also make it clear that once a phase is concluded and there’s an ok to proceed, going back will cost additional budget and change the timelines (for example if they want to make major changes on a final illustration where they approved the sketch in an earlier phase). The worst client is the one that keeps changing his/her mind after the work has been done (and I have clients like that in my day job....they are horrid and can kill your schedule as well as your sanity).
How long each phase take...that depends entirely on you and your process. You should be able to estimate how fast you can think, design, draw, paint....create your magic. And then double that time before you write it in a contract 
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