From Nancy Pearl’s Book Lust, the Rule of 50 will change your life:
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.
When to give up on a book you're reading