hmmm. that's a good question. It's been my experience that it's about 2-3 gal on previous vehicles.
The best way to find out, would be to immediately top off the tank with fuel when the light comes on, and then subtract that from the total capacity of the tank. Then regardless of the "typical" you should know what your's is at the very least.
Are you aware that running the tank that low causes the fuel pump to get very hot. The pump relies on the fuel in the tank to keep it cool. If it's run like this too often can lead to premature failure, and cost you down time while it gets fixed.
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