All we really need to know to answer this one is the speed of sound, and that's not easy. Sound has different speeds in wet air, dry air, warm air, cool air, and air at high or low pressure ... and ALL of these are present on the way from the thunderstorm to you !
So let's just use the number for the speed of sound in dry air at sea level and comfortable temperature: 343 meters/sec.
time = (distance) / (speed)
Time = (1485 meters) / (343 m/s)
Time = (1485/343) second
Time = 4.3 seconds roughly approximately around about
