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I'm pretty sure the manual states (for non-SRTs) to not let the gas go below 1/4 tank if you can avoid it. It does put more stress on the fuel pump from what I'm aware. I typically fuel up when the light goes on.
Once the indicator lamp comes on you have approximately 2 gallons of fuel left. Figure you can get between 16-25 mpg depending on your driving situation so you should be able to travel 32-50 miles more.