
Ebay recommendations are that you wait seven days for payment and, if not forthcoming, open an 'unpaid item' dispute on the eighth day. You cannot do this any sooner than the 8th day so it is pointless putting 'payment required within x no. of days' as it is unenforceable. If the buyer does not respond to the dispute (I think they have a week to do so) you can close it and you will receive a refund on your final value fees. I would not relist the item until I was sure the dispute was closed as, if the buyer decides to play ball at the last minute and you have sold the item to someone else, they can report you as a non performing seller.
I usually send one or two reminders and open a dispute around 10 days after the end of the auction...I think that generally gives enough grace in case there is a genuine reason that the buyer is unable to respond sooner.