It may not be said directly, but there is not a time limit put on deteroration after PEM in most guidelines, I think. I don't think they insist that PEM is always short term and people always recover to their previous level.Which criteria are you referring to?
And the disabliity scales used for ME (Bell scale and MEA scale both include very severe ME with no indication that this is necessarily temporary.
I was really thinking more of the NICE guideline sections on PEM and relapse, and on severe and very severe ME that make it clear deterioration can be long lasting.