Let me add some of my experiences of aircraft condition and put IFixPlanes last comments into pilot speak.
One of the airtest checks is to select each pack off one at a time at max alt to verify that either pack will maintain cabin pressure. Then we switch both off we measure the “leak rate” on the cabin VSI. The acceptable limit is 2000fpm, or 2500fpm on short body a/c.
With new build a/c the test is a non-event but with in-service a/c the results can be alarming. I have seen some aircraft that gave a leak rate of almost 2000fpm when a single pack was switched off – this “single failure” would give you about 3min before the PSUs drop. I have also seen a/c that have a leak rate of over 4000fpm when both packs are switched off.
The above figures were of course out of limits and the aircraft were rectified, but they and many others (yours!) could be flying in a similar condition and you won’t know it. The law allows continued flight above FL250 if a pack fails in-flight, but you should be aware that your aircraft may not perform as well as a new aircraft. From max altitude, if the cabin descends at 4000fpm you will only have 90secs before the rubber jungle deploys. How long would it take you from recognising the problem to reach FL100 from FL400?
S&L