When the red battery light comes on - alternator is on way out, how many miles have you managed to drive?

In a G40, I'd personally not drive anywhere with the battery light on.

The ECU needs at least 12.5v in order to operate correctly. A battery light would suggest you're getting less than that, or even worse, more than 15v.

In a queue with headlights on, 2.5 miles. For me it was the alternator plug cable falling to bits... and breakdown guy did not appreciate my lowered car.