Given the popularity of Nike products, certain styles and sizes sell out. Restocks can happen at any time. Be sure to check Nike.com often or search for the product at a nearby Nike store.
Nike doesn't release restock schedules, but you can follow @Nikestore on Twitter for restock announcements. If the product or size you select is sold out, you will receive an out-of-stock message. Items added to your cart are reserved for a brief period of time and may sell out if you don't complete the order in the allotted time.