Several hundred miles at least, and several heat cycles (tire allowed to cool down completely). Also, it's the middle of winter, why are you driving with summer tires? I realize it's probably been warm in NY too, but mornings are cold, and really, the tires are not reaching the same temperatures they do when it's in the 50's-80's outside.
Aside from that, did they road force balance them? Michelin's tires are usually pretty decent. In fact, when I run them on the bike, I don't need any weights at all, or very small (1-2 squares).
You mention they're on the rear, what's on the front? If the level of grip of front vs rear tires is drastically different, your car will handle funny.
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