I would tend to ride it out, but it's somewhat situational. If the book is in a series, that's more incentive to keep it free to pull in new readers and hopefully grab some sales on the other books. If the book's a standalone and the downloads are really drying up, well, there's an argument for pulling the plug sooner and getting it back on the paid lists.
The other thing to look at is whether you've had any free site mentions today. Even small sites can add up as people check their email, FB, etc. later in the day. But if it's been all quiet on the Google front, then you might cut it off.
On the other hand, if there's nothing much to lose by keeping it free, you may as well let it run and take advantage of its rank while you've got it. #93 free ain't bad.