The S&P 500 index fund in my 401(k) is crashing and burning. It is down over 10% for the year...the NAV is down to about $9.
How low do I let it go before I buy more of it?

Timing the market is a very dangerous game. The lucky win more often than the smart. If I knew how to time the market, I certainly would not be working today! I'd've taken $100 and parlayed it into several billion, and I'd be sitting on my own private island somewhere.

The answer to your question can only be found inside of you. What is your comfort level? What level would you be happy with owning more shares of that index fund? If you invested today, at $9.00, and the value dropped to $7.00, would you kick yourself for getting in too high? Alternatively, if you didn't invest today, and the value skyrocketed to $12.00, would you kick yourself for sitting on the sideline?

Following the day-to-day gyrations of the stockmarket is a sure way to get an ulcer. Planning your investment strategy on current price, and not on a comparison between price and underlying value is a certain way to get slaughtered.