A buyer has 4 days (96 hours) from when the listing ends to pay for an item, - because you cannot open an 'Unpaid Item' dispute before that time. Most sellers wait until the end of day 3 and send a pleasant reminder. If still no response from the buyer, they open the dispute on day 4.
A buyer then has a further 4 days in which to pay, and if he does not, you close the dispute, get your fees back, relist the item, the buyer gets a strike on his record, and no one leaves feedback. This here link will tell you all about it:
http://pages.ebay.ca/help/sell/unpaid-items.html