To give you an idea, for the -300 assuming the structural integrity of the aircraft is ok then the max rate of decent from FL250 with the power levers at Flight Idle and Condition Levers at MAX is, I think 214kts (Vmo - max operating speed), then you must reach FL140 in 3 minutes, which I believe is the limit. Hence no oxygen masks. I do believe they're optional as some operators take them out to save weight (including the APU). Obviously crew oxygen supplies are always carried.
If structural integrity is in doubt then rate of decent is limited to 190kts, or if you're at FL180 or less then a decent of 2000ft per minute is sufficient.
That's my limited understanding......not sure about the Q400 though.