The precise definition will be a part of the certification of your aircraft by the issuing authority, (in your case Malta?), and may vary a bit from country to country.
On the registers that I have operated aircraft the 90 seconds has been timed from the time the order was given, either by the tech crew, (if still able), or the senior surviving Cabin Crew member until the last able bodied pax and crew were out of the aircraft.
