With my brand-new 2015 S Premium, I was keen to see where it was when the low fuel light came on. And when it did, I had to drive about two miles in city traffic to get to a gas station, and I had a bit more than 2.7 gallons left in the tank. At 35 MPG, that's about 95 miles before it would completely run out. So, I'm figuring that I can easily go 50 miles on the freeway to get to a gas station before I would need to start sweating.
Regarding how much might be left when it actually reads "Empty," I probably won't ever go near that. I don't know that you really need to fill up at a quarter of a tank remaining, but I'll certainly fill it up at my earliest convenience once the low fuel light comes on.