How long can a writer be away from a work without losing momentum to finish it?
That depends, of course, on the writer and the work. It also depends in part on the writing method. If you know exactly what comes next in a novel -- in other words, if you're an outliner -- perhaps you can leave a work for months and then pick up where you left off with no trouble. Or perhaps not -- I'm no outliner, and so wouldn't know. My working method (and I hesitate to dignify it with that term) consists of feeling my way through a novel by a combination of (1) becoming the characters to figure out what they will do, (2) visualizing no more than two scenes ahead of where I am now, while simultaneously craning my metaphorical neck for glimpses of some eventual end, and (3) blind luck.
This method does not lend itself well to leaving a novel-in-progress for long periods, and I have been away from mine for over six weeks. A week to prepare for Clarion by reading, line-editing, and wiriting critiques of student stories; one week teaching Clarion; a week to prepare for Taos Toolbox; two weeks of teaching at Taos; a week of picking up by normal life and writing several neglected small commitments (an Appreciation of Connie Willis for the WFC program book; proofing a book for e-Pub; stuff).
Six weeks is too long. I have lost the momentum, forgotten where my complex cast of characters each is located and what they're doing, slipped out of identification with my heroine. So I've had to do what I never have done before with a novel: start over. Each chapter must be read, thought about, revised. Slowly the book is coming back to me. Again, this is not just a matter of mental reminder, but of emotional investment.
It causes me to question, though: How do writers like Connie Willis and George R.R. Martin, who write a novel over a period of YEARS, manage to do it?