This article is from the eBay FAQ, by William B. Shaw firstname.lastname@example.org with numerous contributions by others.
Technically, you can relist the item if you haven't heard from the buyer in
the three business days. Most eBay users would probably agree that this is
MUCH too soon. It's not uncommon for a buyer to send a payment without
contacting you first. EBay recommends you send a payment reminder between
three and thirty days from auction close.