As a Medford resident and therefore the proud owner of a restricted LTC, when this inevitably goes to SCOTUS and is affirmed, I wonder if they will give states like MA a grace period to change their laws (like with stun guns) or would all state carry permits become meaningless relatively immediately?
SCOTUS didn't give a grace period. They kicked it back to the SJC with a short
per curiam opinion basically bitch slapping them for brazenly ignoring Heller and misconstruing it as not applying to stun guns. They left it up to the SJC to make a new decision in light of their ruling that stun guns were protected by 2A. The SJC sat on it for over two years and then gave a 60-day "grace period" on top of that. But the grace period was a legal falsehood. You could buy a stun gun out of state and carry it on you in MA during that period and even if they arrested you, they couldn't convict you.
The way this would likely work is, a test case from some state would make its way to SCOTUS. SCOTUS would rule on that law under 2A. That ruling would apply directly to that state's law immediately, at least in the sense that criminal prosecution would be barred. What would happen in other states could go a few different ways.
If the decision was full-on judicial activism, like in Miranda v. Arizona, it would have instantaneous nationwide applicability, but that's highly unlikely.
If another state had very similar laws, other states could just stop enforcing them. That's what happened with abortion after Roe v. Wade, and that's why MA still has had an abortion ban on the books after all this time.
They could also refuse to back down, or even go out of their way to try to pass new full retard laws they think would comply with the letter of SCOTUS's ruling but destroy the spirit. That's what happened with Brown v. Board of Education in the South, they called it "massive resistance," and it took decades to sort it out. People in each state had to file lawsuits in the federal district courts, and slowly, gruelingly push things forward against hostile state legislatures that were constantly putting up more sophisticated barriers.
The last option would be states trying to narrow the scope of the laws in good faith to comply with the ruling before anyone files any lawsuits to strike them down.