Asking users to input that manually seems unrealistic. I read a lot, both for school and for pleasure. I also listen to many audiobooks and lectures. I couldn't tell you offhand how many minutes long a book I listened to recently was, or the exact number of pages I read. I would probably skip filling that out every time. If I was forced, I'd feel irritated, and would likely try to input some bogus information.
When it comes to digital files, you could find out the answer pretty easily. I currently use iTunes, and it tracks when I last played a file, how much of the file I played, etc. The Kindle also remembers which page I accessed last so I assume that's stored in a file somewhere. I'd be happy to share that data if sharing it benefited me in some way.
Paper books are more difficult. However, although I often couldn't tell you the page number, I can more frequently remember my overall progress (25% finished, half finished, etc) which can be translated into pages. If you know a book is 500 pages and the person says they're 25% finished, just run a calculation on that and figure out the approximate page.
I don't think it's a bad idea to have a way to input a specific page number for people who do remember and want to do that. However, I use goodreads.com currently... the only option is to enter a specific page number, and since I rarely know what that is, I don't use it. I don't care enough to walk across the room and find the book and look for the bookmark and realize I have to flip through the book to figure it out, etc.
People who don't read much might have an easier time estimating their durations. But going through 20 books / audiobooks and manually figuring out exactly where I am in each book sounds like a chore. If your tool is intended for people who don't read regularly, it'd only require occasional effort. But I'm a student, so I read large amounts daily from a variety of different books and it'd quickly get confusing and frustrating.
My preference would be sharing my data. Apart from that, I'd be more likely to use a tool where I click "start timer" the moment I start reading and "end" when I'm finished. It couldn't track my page count in a paper book, but it could get an idea of the duration. If I estimated the amount completed later, it could start comparing that to the duration and getting an idea of how many pages I usually read in an hour, etc.
