Just considering word count will not give you a realistic estimate of what is needed to edit a large document. Other factors to be considered are the type of writing, what kind of editing is desired, and the style of editing that needs to be used.
What is the word count?
Due to different fonts, sizes, and graphics, the page count for a document might not be reflective of the actual work involved. The word count is more accurate.
Is the document fiction or non-fiction?
Fiction and non-fiction are reviewed differently. Fiction documents are reviewed for plot, character development , and setting. Non-fiction is reviewed to make sure it is understandable, is organized, and flows well.
What kind of editing/proofreading is needed: basic proofreading, copy editing, formatting, more substantiative editing, or ghost writing?
Proofreading checks for typos, punctuation errors, spelling errors, and other inconsistencies. Copy editing involves the proofreading plus some grammar, word usage, capitalization, and word breaks. Substantiative editing includes copy editing and making the sure material flows well, is organized, and is developed well. Formatting involves reviewing the formatting for consistency, checking formatting to make sure it is appropriate, and reviewing page numbers, margins, indentations, and citations. For ghost writing some notes, rough draft, or dictation is provided and the proofreader/editor actually writes the material.
Is there a particular style of editing needed?
Some common styles of editing are Chicago, MLA, and APA. MLA stands for Modern Language Association and is typified by sources being cited in the text in parenthesis. The APA format is specified by the American Psychological Association. The APA format is known for having brief citations in parenthesis in the text followed by a more detailed reference list at the end of the document. The Chicago Manual of Style uses footnotes.