Depends on the length of the book, mostly. 25 pages of books up to 300 pages in length. 50 pages of books from 300 pages up to 500 pages in length, 10% of books beyond 500 pages in length up to a maximum of 100 pages of a 1000 page or longer book. Also, up to 3 days after I lay the book down. If I don't pick up within 3 days, I'm likely not going to continue.
Still there are exceptions. I have picked up books beyond the 3 day limit. I have also read up to 200 pages of a 400 page book before throwing in the towel, however, most times neither of those things happen. And I have also finished reading some books and then thought "Why did I waste my time on this drivel?"
One of those exceptions was Robert Heinlein's Stranger in a Strange Land. I read almost exactly half before I gave it up as being entirely too slow. To be fair, I established my "approximately 10%" rule more than 20 years after that. Yet, I love other books Heinlein wrote, having read many of them two or three times.