How Do You Know When to Stop Expanding and Start Revising?

The relationship of writer to book-in-progress reminds me of a marriage.  As opposed to a date. 

Poems, articles, columns, and short stories are all creative commitments, sure.  But  even if they linger unfinished for a while, they are short relationships compared to 350 pages of manuscript. 

With a book, you regularly re-evaluate your progress, your pu…