"Old age comes on suddenly, and not gradually as is thought." Emily Dickinson
Lifehacker cites the post Austin Kleon cited when he got me started on the Rule of Fifty. The comments at Nancy Pearl's blog offer some interesting thoughts and variations: the severe "my first-page test. If the author hasn’t grabbed me enough to make me turn the page, I’m outta there" and Pearl's own “rule” for kids and teens and chapter books is three chapters".