Illustration of a book is a whole process, and each step of the process can take an indefinite amount of time. Ultimately it all depends on how detailed and specific your illustration needs are. The more descriptive your illustrations are, the longer they will take. Additionally, some particular art styles also increase the deadline of the project. These include realism, horror, or any custom art style which the book author may demand.
The steps for creating an illustrated artwork are as follows:
Initial Concept
Development
Sketching
Refining
Coloring
Review and Feedback
Finalizing the artwork