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I would think the next day, if not a few days after. It's already available online would not be wise to still have the old ones in the stores. Would confuse people.
 
Just got back from my local Apple store, said they have them now but they will only be out on the floor tomorrow.
 
Store size and demographics of the area seem to play a big role. The local stores (3) near me get lots of new product in far more model configurations than the stores in outlying areas do.
 
Out here in the hinterlands of southeast Michigan, we had to wait a couple days for the Retina models to show up in our local Apple stores. In fact, I went to the Somerset store the day after the announcement and was told they weren't there yet. I went back the next day to see one.
 
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