mouthbreathing is *the* cardinal sin of mewing. it causes the jaw to swing down and back,  the teeth to stop being retained and aligned by the lips, the tongue to fall off the roof of the palate (in order to breathe) and the head to come forward in order to open the airway.

you would likely see many problems manifest instantly such as all that come with mouthbreathing and see actual craniofacial damage within months (even weeks on a very small scale) . I personally know of a man in his thirties who used to be handsome, got ill for an extended time and couldn't breathe through nose and become ugly over the course of a few months. he stayed ugly because he couldn't adjust back to nasal breathing. genuinely tragic. 

idk how long it would take for the damage to be really profound like you say (probably a few years) but I know for certain that the damage begins to accumulate immediately and consistently when you stop mewing 

i

AstroSky did a Youtube video about Brad Pitt's presumed tongue posture. He determined that his tongue posture was actually not ideal, and that his maxilla / midface is not very forward grown, and that his lower third is great because he probable chewed a lot since young.

Dunno, I see people with good jaws who hunch their neck when they look at their phone.

My supposition is that once you earn it, it's pretty much yours for life. As long as you at least maintain nasal breathing and a decent posture