Like many things, it is an “it all depends!” answer! Is it relatively clean? Are there many corrections to make? Have I read the manuscript before? What’s the genre?
I’d say it would take 10-15 hours of editing, then another 3-5 hours of polishing and then writing the letter. For me, I’d spread it over a couple of days. Usually I’ll do the edit, let it sit for a day or two, then the final review/letter and send it off.
Of course, it will also depend on other projects that are due and the daily business stuff! And, like authors, editors can be working with manuscripts at different stages, so looking at proposals, editing and then doing a second pass of a developmental or line edit or copy edit!
That’s why we recommend “saving” a spot two to three months ahead and be clear about your deadlines as well. And building in extra time whenever possible!