In real estate, "time is of the essence." For the protection of both buyer and seller it is best not to have offers open ended. If a seller drags his feet in responding, a buyer may continue to look at other properties and ultimately withdraw the offer, particularly with so much inventory on the market. Buyer could then proceed with a new offer. Again, it is best to keep the offer moving forward for the protection of both parties. A seller might entertain another offer from another buyer, if there is no timeline in the offer.