It is within AMM Limit when you lost up to approximately 1PSI within 60 seconds without any air source and all valves closed. :ok:
With one perfect working pack it might be possible to maintain the ∆p at FL400, but is your structure in perfect condition?:E

I dispatched with a 1 pack operation restricted to FL250 one day and on descent with the flight attendant asking what we wanted to drink was awarded with a take off warning horn. I looked at the copilot because he had been pulling tricks all day wondering how he did it. He looked confused so I thought, WAIT! that sound is for loss of pressurization too but I had never heard it. Looked up and sure enough, the cabin was above 10,000 and climbing. Putting on the bad pack fixed it so we didn't have to add power to the working side. Some cooling equipment outflow valve had stuck open letting the air leak out. No big deal but it sure gets your attention. I am happy we didn't get the problem enroute and continue to a higher altitude as is allowed.

...and people ask me which operator I fly with...

Sometimes MEL requires not using a pack for reasons that only occur on take off at high power settings and works fine at cruise or descent. You know why they MELed it so sometimes you know it is fine in the regime of flight you are in so you can consider using it if another failure happens. Most pilots use common sense to decide if they should use the option of using a system that was not MEL available but was operationally usable.

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