Most trades on AmiBay are concluded within a 24 hour period, however in the extreme case, if you have not had a message from an interested party within 24 hours then it would be only be fair for you to contact your buyer one last time. Should that you still not receive a response within a reasonable amount of time (use your better judgement) then you should move onto the next buyer in line.
You should send a final message to the prospective buyer informing them that you are moving on to the next in line as well as update your sale thread.
Of course you are welcome to wait longer if you wish. If you do, you should inform your other prospective buyers in your sale thread.