I'd argue that "As slow as possible" is a note to the performer, not an absolute statement. Play it as slow as _you_ feel you possibly can. For some performers that's an hour, for some it's 8, for some it's 24 hours, and for this particular performance it's 639 years.

A performer may think they can play for 8 hours, but if they can do that they can also likely hit 8 hours and 1 minute…

So you could interpret it a slow tempo you can definitely finish, a tempo you might finish, etc but actual limits are not really predictable.