Correspondingly, how long should dough ferment?
Dough ferments at room temperature for 6 to 12 hours or in the refrigerator at 39°F to 42°F for at least 24 hours and at most 48 hours.
Furthermore, how do you store fermented dough? Allow the dough to warm up a little before baking. Just to add to other answers, it's often easier to refrigerate for the first proof. That is: mix, refrigerate for a "first rise" (from a few hours to a few days), then remove from fridge, shape, and then let rise the second time until read to bake.
In this manner, can you ferment dough too long?
If you want to let you dough proof for longer, try bulk-fermenting it in a cooler place, but don't allow it to go longer than three hours or structure and flavor may be compromised.
Is over fermented dough safe to eat?
Almost certainly, especially if you bake it. Your dough doesn't contain anything that will "go bad" in 15 hours at room temperature. Chances are the dough will taste a bit odd after baking -- overly "yeasty" or "beer-like," with some "off" flavors. It won't be completely inedible, but it probably won't taste great.