It's specced at a MAX of 20mS. Actually you can go longer as the brain in the servo is a sample and hold based on the pulse HI period only. I've used up to 40mS with no problems. If you go much longer than that the servo brain goes into low power "limp" mode and powers down its hbridge.
You can go LESS than 20mS with no probs, I made a 4 servo system that used 2.5mS events to sequence the 4 servos so the total period for any 1 servo was 10mS.