As the originator of a normal contract, the buyer usually specifies within the document the period of time the seller has to respond to your offer. As previously mentioned, under normal circumstances, 24 hours should be an ample amount of time for them to respond to your offer.
On the other hand, if you are dealing with a bank in a short sale or foreclosure there can be little regard on their part for buyer established deadlines.
I will assume you have not made an offer yet. When you do make an offer on the first page of the agreement of sale you write in the requested reply time. If the seller does not get back to you within this time frame you may pull your offer and get your deposit back. I have never seen this done but it does keep the seller's agent on the ball. Let's face it the seller's already know what their numbers are before they listed the property so it's not like they have no clue.
Don's comments below are correct it all depends on who the seller is. But, if it is a seller "person" or Estate Property I would request a 24 hour reply as you do not want other buyers to get in with their offer. In most cases, the offer is negotiated over the phone and the final document is signed within 2 to 3 days. If the answer is taking a few days from the seller's ... it is important that the your buyer agent make it clear with the seller agent that since it is taking a few days on the seller side you want to find out the position of the seller if other offers come in during this process and how they will be handled. Some seller's say once the agreement has been negotiated that they will not accept reviewing other offers...but legally they can keep accepting offers even after your offer has been signed which would be held as a backup offer, in case you do not go to settlement.
The important part of getting all the necessary agreements signed is communication and regular followup between the Buyer Agent and the Seller's Agent.
Foreclosures & Bank Owned Homes: Average 7 to 14 days.
Short Sales - Average 30 to 90 days
Consumer Sellers & Estate Properties:Average 2 to 3 days
Your agent should be aware of the sellers situation and inform you of when to expect a response.