It's the same with us in respect of how long to keep up the command, however, we use the simple command of "Brace! Brace!" Having worked for an airline that used, "Get your heads down, get your heads down!", I have to admit to finding our command much easier to keep up at high volume for what could be a fair amount of time. We also do it in tandem from front to back and/or side to side, depending on aircraft type so you'll hear the commands alternately from the left and right, back to front. Again, that gives us half a chance to keep it going, while grabbing a breath in between. Having said that, in all honesty, I'm not entirely sure it matters what you shout or whether the passengers will even be able to make out what you are saying in true emergency. I'd have thought the most important part of any emergency command would be to keep it simple and direct.
As far as adding directions to an evacuation command, while I agree with what has been said so far, this is one of my bug bears in the current company. The command in such circumstances is "Evacuate, evacuate, hazard on left/right.." If part of the message is lost in the general melee, all you hear is "Evacuate......Left" Which way are you going to go? We cabin crew have always been taught to only ever give positive commands for that exact reason. I did try to very politely bring this up after a mock up scenario but was shot down in flames by a training Captain.
