To estimate how long a thunderstorm will last, meteorologist can use the formula t=✔️d^3/216 where t is the time in hours and d is the diameter of the storm in miles.

t=(512/216)^(1/2)
t=((8)(3)^(1/2))/9
t is about 1.539600718 hours 

I don't believe the storm will be twice as long because the number multiplied by itself three times will give a much, much higher number than what was your original diameter.