For an average person, the time of useful consciousness at 25,000 feet is about three to five minutes. At 35,000 feet, it’s only about 30 to 60 seconds. That’s not much time to don your oxygen mask and descend to a safer altitude, especially if the cockpit is obscured by fog or mist due to rapid decompression. Some pilots have run out of time, with fatal results. Currently, if you’re transitioning to a pressurized aircraft that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 feet msl, you must have a high-altitude endorsement. According to 14 CFR Subpart A, 61.31(g), you’ll need both a ground sign-off on the physiological hazards of high-altitude flight, such as hypoxia, and a logbook endorsement showing successful flight training. Not surprisingly, this training includes procedures for emergency descents.