When you make an offer, there is a clause in the standard OREA form to make it irrevocable for a specific period of time.
"This offer shall be irrevocable by (Seller/Buyer) until (time) on the (date) day of (month), 20xx, after which time, if not accepted, this offer shall be null and void and the deposit shall be returned to the Buyer in full without interest."
Until that deadline has passed, the offer can not be changed or withdrawn by whoever made it (typically the buyer first, but this applies to counter-offers from the seller to the buyer as well). The party to whom the offer has been made has until that time to accept it and communicate acceptance.
While not technically required for a contract in general, the nature of real estate makes this a pretty standard part of the process. It gives the seller assurance that the buyer will be bound but also gives time to consider other offers that might come in. And it gives the buyer an idea of when to expect an answer, not held in indefinite limbo while a seller considers it and possibly waits for other offers to come in.
On that note, a shorter time period can put pressure on the sellers and is generally better for buyers, while a longer time period works in favour of the sellers as they can bide their time a little to see if anything else does come in. In practice, it's a balancing act between giving what is necessary and trying to protect your own interests as well. The listing may ask for a 24 hour irrevocable, but the buyer is not strictly required to oblige that request, just like they're not required to give full price or any other terms outlined in the listing.
But then, the seller may be from out of country, in hospital, or any of a host of other possible scenarios that might require some time to respond, and a shorter deadline may not be feasible. You might just be shooting yourself in the foot and irking the seller for no reason. And even if there is no 'good reason', it's still one thing the seller is looking for that you're not giving them, so it all has to be considered strategically and with an overall view of the rest of the offer.
Make sure you work with a competent and experienced real estate professional, and trust their advice.
