Nancy Pearl, the now famous librarian, offers this rule for reading.
Believe me, nobody is going to get any points in heaven by slogging their way through a book they aren't enjoying but think they ought to read. I live by what I call 'the rule of fifty,' which acknowledges that time is short and the world of books is immense. If you're fifty years old or younger, give every book about fifty pages before you decide to commit yourself to reading it, or give it up. If you're over fifty, which is when time gets even shorter, subtract your age from 100. The result is the number of pages you should read before deciding.
You may have seen on the sidebar that my current book, for a while now, has been Claire Tomalin's biography of Thomas Hardy. My scheme this year was to read it in the month of Hardy's birth (June). At first I was quite smitten, but as the days went on, I found myself less and less drawn to it. As I was reading the other day, my mind wandered to Nancy Pearl's rule. I looked down, and incredibly I was on the page number which one gets as a result of subracting my age from 100. That was a good enough sign for me to quit, at least for a while, and maybe permanently. I thought I was going to be fascinated with the subject, but I found it a bit dry. I got bogged down in details. I'd read for five minutes and then think of something else to do. Sometimes I think I'm just busy when I don't read much, but soon enough I realize it is the book not the time. There is always time to read, and when I love a book I can't stay away.