Mine comes on around the same time. It comes on when you have about 60 or 70 (can't exactly remember) miles left on a tank. I had a 530i and it would come on at the same miles-to-empty also, but that was at less than 1/8th tank. Of course the 530 got around 22MPG around town, and this M5 is getting around 14MPG.
You can press the upper button on the turn signal stalk to clear it for the current drive unless you get down to 30 mile left then it comes on at every mile until you fill it.