You earn advertising fees on any qualifying items placed in a customer's Shopping Cart within 24 hours of their arrival at Amazon via your Associates link. However, this 24-hour window closes once the customer submits his order or reenters Amazon through another Associate's link. Once the window is closed, you will not earn advertising fees on any subsequent purchases. However, if the customer then returns to Amazon through one of your Associates links, this opens a new 24-hour window.
It is of course possible that a customer may arrive at Amazon via your Associates link, add an item to his Shopping Cart, and then leave Amazon without placing an order.
As long as the item was added to the customer's Shopping Cart during this 24-hour window, you will still earn a referral fee if the order is placed before the Shopping Cart expires (usually after 90 days). The referral fee will not be credited to your Associates account until the customer has purchased the item, accepted delivery, and remitted full payment to Amazon.