It happens when it happens. For NaNoWriMo novels, I always come up with a working title about 10,000 words in, just something to refer to it by while I write it. My working titles usually have nothing to do with the story, and always have something to do with food -- so far I've used Ketchup, Toast, Bagels, and Tahini. Some of these I've renamed, some I haven't. Bagels became
Some Say Ice, inspired by the Robert Frost poem
Fire and Ice. Tahini is currently
A Treatise on Some Small Dark Spaces of the Mind, although I'm considering changing it again. Ketchup and Toast have, so far, retained their working titles.