How long after stroke can one distinguish short term (reversible) /permanent (irreversible) damage?

Stroke: There isn't a simple answer. Some strokes appear likely to be irreversible from the start but improve notwithstanding. Healing of nerve damage can take many weeks and therapy can also modify what's going on so a simple answer that fits all situations isn't possible.

2 years: The sooner there is some return of function, the more complete the return is likely to be.