Once you're very familiar with the process, you could get a draft of a basic prose novel ready for proofreading in a few hours. Then it has to be proofread and completed.

Beginners, and people working on more advanced books, can take much, much, much longer.

it varies widely depending on the length and type of book and how much free time the volunteer has to devote to it

Anywhere between 1 week for the simplest (straight narrative, not too much verse or endnotes) and ~1 year (thousands of endnotes, pages of verse, drama, in-line references to book titles, use of technical terms, etc)