I usually send a polite reminder on the second day then one more a day or so after. I don't sell loads on eBay but this has worked so far.
But an item sold in a bidding frenzy and I suspect the successful bidder won't pay as it did go for more than I was expecting, and there is a BIN of the same item cheaper atm.
Would it be harsh of me to open a non-payment case as soon as allowed? I have sent an invoice reminder with polite note. It is a popular item and I'm sure it will sell.