I hold items for 48 hours maximum....that's after someone has said that they are definitely interested in purchasing the item. I sometimes (but rarely) make special arrangements on a case-by-case basis if they make plans to pay on a specific date (like if they get paid on Friday, they'll pay on Friday). If it's past the date they say they will pay, I send one follow-up PM to check up. If I don't get a response from THAT in 48 hours, the item goes back up for sale. Generally there's a response by that point, but if the person has been on and is ignoring the PM, I'd be inclined to take that as "No, I'm no longer interested".