It varies, I will shoot at anywhere from 1 sec intervals to 10 sec intervals and often the shutter time varies throughout the shot (my custom intervalometer can ramp the camera shutter in bulb mode to assist with day to night shots...so a shot may start out at 100 ms and end at 10 seconds, I do usually try to keep the interval static though). I think I did most of those shots at 3 second intervals or so...the fireworks were at 1 second intervals
-Tre