Last time I did this I ordered an Air for store delivery since I was out of town. I believe they sent me a reminder after three or four days that I had to come get it or they would cancel the order. I called the store and explained I was out of town and they agreed to keep it until I returned a few days later.
I don't recall seeing anything in writing when I ordered explicitly saying how many days it was though.
Couldn't make it to the 2-hour mark before bumping, huh?
There is a defined timeline which I can't remember exactly but which you can get by calling Apple. I recently picked up two new computers that were arriving on different days. I think I was told that if the machine came into the store on Monday or Tuesday, they wanted it picked up by Sunday.
But when I called the specific store itself, they told me they didn't go by such a stringent deadline and wouldn't cancel the order and return the shipment without reaching out to the customer first.
So, in answer to your question, there is a finite, defined schedule put forth by the corporate office, but your mileage really depends on the policies of the specific store receiving the shipment.