[MisterBiggs]

Using a pretty simple atmospheric model going from the original 1,325 km to the newest 540 km altitude, the air density doubles (1.990x).

Then there's the increase in velocity since the orbit is now smaller, which is about 300 m/s.

So overall the drag force doubles, but there's no way to tell how long the lifespan is without knowing a coefficient of drag and using a much more complicated atmospheric model that takes into account things like Sun and geomagnetic activity.