I got to participate in one of these things for a 737, many many years ago. It was a new plane in the fleet, so the company had to certify all the equipment and they had a new class of FAs, so they just had the new FAs try to empty a (mostly) full 73- in under 90 seconds. Then we got to blow the safety rafts, too...that was the fun part.
It was a little cheesey, though..."sit here until everyone starts yelling, then go row by row to the nearest exit." Cause I'm sure that's how it would actually happen.