The only real "rule" I know of is that after 2 days, a seller can open a unpaid item case against a buyer. Not sure too many do that though as most (including moi) let buyers hold off till they are finished over days, even weeks to pay one invoice and combine shipping.
I have opened several cases simply to get a buyer to pay and get the item(s) on their way when they've had zero activity over a couple weeks and/or they are notoriously slow payers.
I sell a ton but rarely buy on
ebay but as far as I know there is no penalty for slow payers.