I think that if a buyer pays you quickly and it is a no hassle payment then you should leave feedback asap,and then tell the buyer when you send them their item,that you left feedback and that you would appreciate the same if they are happy.I always check to see if the buyer has a history of leaving feedback.If they do...then they get it quickly from me as long as I don't have other things going on at the time,and that's when I might leave it a few days.
This "eye for an eye....give me feedback or else" idea sucks ass.
If you don't get any feedback in return,then so be it.
I came across a seller the other day who stated that he will not leave feedback unless he gets some.That is so weak,and tells me that the seller does not have faith in his own product.