When do you quit reading a book?

From the wisdom of LaVonne Neff:

A friend gave me this useful formula : Subtract your age from the number 100, and the answer is the number of pages you have to read before giving up on a book you’re not enjoying.* Her advice came to mind the other day as I was preparing to return yet another partially read book to the library. Oops, I thought : am I dangerously deficient in perseverance?

No, I said to myself. Life is short – why spend it with books I don’t really want to read?

Here’s a list of the books I’ve quit reading (or listening to) since June. Every one of them, by the way, is worth reading….

How about you, when do you quit reading?