Someone posted that the 747 depressurizes at 2500 fpm.
From FCOM 1:
Main Deck Cargo Compartment Suppression
The main deck cargo compartment is a Class E compartment. Pushing the Main Deck Cargo Fire Arm switch configures equipment cooling to closed loop and turns off two packs and airflow to all cargo compartments.
Subsequently pushing the Cargo Depress/Disch switch initiates automatic aircraft depressurization at the rate of 9,000 feet per minute to slightly less than aircraft pressure altitude, or 20,000 feet cabin altitude, whichever comes first. Then depressurization continues as required at the reduced rate of 2,500 feet per minute until the cabin pressure altitude is just below the the aircraft cabin altitude, or 25,000 feet cabin altitude whichever comes first.