The seller can ultimately take as long as they would like to answer a bid for thier property. However, it is customary and generally accepted that offers and counter offers should be answered in a 24 hour period or sooner. It is not uncommon for a negotiation on a home to involve several offers and counter offers all happening in a very short period of time. For example the first home I sold was my personal residence and we counter offered roughly 6 times over a period of 4 hours.
If you find yourself waiting for more than a period of 24 hours a short follow up call to check the status of your offer is warranted. Be careful however not to seem over anxious or desperate in your communication. If the seller is serious and professional they should respond rather quickly.
During your negotiation always start with your BATNA. (Best Alternative To the Negotiated Agreement) In other words know what your bottom limit is and what you are willing to accept BEFORE you start bidding. It is very easy to get caught up in the excitement and anxiety of the negotiation causing you to make a descision based on emotion rather than your needs.
Most if not all purchase and sales contracts have certain dates contained therein to protect all parties. One of the dates in the contract is the acceptance date for the seller. Without such a date, a seller could hold onto a contract for an undetermined amount of time and accept it even if the buyer making the offer has purchased another home. And the contract would be binding in theory. Usually, it depends on the seller circumstances. If they are still in the area, a 2 or 3 day timeframe is usually ample time to have a response. The Realtor, if there is one, will generally advise you if circumstances are unusual—a divorcing couple, or an out of town owner, for example—and a longer timeframe might be needed. Remember, that you control the acceptance time, because you are the one making the offer. Please note that a verbal offer is not binding.