Putting the book flat on the glass means that you scan two pages at a time. A reasonable modern scanner will scan the area of two typical pages at 400dpi in anywhere from 20 to 40 seconds--let's call it 30 seconds for two pages. That's four pages a minute, or 240 pages an hour. You could reasonably get through a 400 page book in two hours, even allowing for an occasional break or glitch.
Of course, you should also allow time for scanning a few trial pages with different settings before you start, to decide which settings to use. Ten minutes spent here can save you hours of proofreading time.
There are two big tips that can save you a lot of scanning time:
If your OCR or scanner control package has a timer setting, that automatically keeps scanning without user intervention, you can forget about the screen and just keep turning the pages as needed.
You should set your scanner just to scan the area the book covers on the glass. By default, your software will probably scan the full area of the glass, and usually, your book won't need that. By scanning only what you need, you may typically save anything from 20% to 70% of the time taken to scan the full area. If your book is small enough to open flat across the scanner instead of "down" the side, 400 pages an hour is not out of the question with this trick.