Emergency Descent Cabin Altitude Requirements
The FARs call out two basic requirements for maximum cabin altitude:
1. Never above 40K feet.
2. Time above 25K feet shall be less than 2 minutes.
The first one is usually met by assuming a "reasonable hole size" - about the size of a single cabin window. With airplane ceilings at about 43K feet, it does not take much of a descent to satisfy #1.
The second requirement above is the tricky one. The airplane must essentially be able to descend to 25K feet true altitude in two minutes or less. The proceedure is speedbrakes up, idle thrust, push over to accelerate to near Mmo. Transitioning from Mmo to Vmo is a bit of a challenge as it requires pulling the nose up several degrees. Descent along a line of constant Mach number involves accelerating in terms of true airspeed. Descent along a line of constant Vcas involves decelerating in terms of true airspeed. Fortunately the Vmo/Mmo corner altitude is not much above 25K feet for most models so once you are down to the point where you reach Vmo you are almost down to 25K.
Another consideration is that while emergency descent rate is slowest when heavily loaded, a heavy airplane will be cruising at a slightly lower altitude than a lighter one and thus does not have as far to go to get to 25K feet.