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Any time frame MAY be realistic. When you get your car depends on when the dealer gets an allocation for a blue Z. His allocation depends on his past sales history, so bigger dealerships get more cars. Dealers can get a report which predicts their allocations for the year. Assuming your dealer is being straight with you, he probably expects an allocation for a blue Z around the time predicted.
That being said, if this is a really small dealer who won't get many Corvettes this year, and he just wants to lock you into the order, it could be much longer than that. It really depends on how honest the dealer is being with you.
The fact that made me move from a smaller dealer to a larger one, when I ordered my car, was realizing that a dealer can place an order request for any car they want, but there's no guarantee that they'll actually get that car. Whether they do or no depends on their allocations.
Hope this isn't totally confusing. Chances are your dealer is being honest with you.