When I start reading a book while I'm working at the library it has three pages to completely hook me. If it hasn't woven me in by three pages I put it down. I have work to do! There are items to shelve, patrons to help, machines to poke, cats to feed, and more books to try three pages of!
When I start reading a book while I'm at home it has 300 pages to hook me. If it hasn't hooked me after 300 pages I put it down, usually because it's over.