Offers are not open-ended; they lapse after some period of time. An offer may contain its own specific time limitation—for example, “until close of business today.”
In the absence of an expressly stated time limit, the common-law rule is that the offer expires at the end of a “reasonable” time. Such a period is a factual question in each case and depends on the particular circumstances, including the nature of the service or property being contracted for, the manner in which the offer is made, and the means by which the acceptance is expected to be made. Whenever the contract involves a speculative transaction—the sale of securities or land, for instance—the time period will depend on the nature of the security and the risk involved. In general, the greater the risk to the seller, the shorter the period of time. Karen offers to sell Gary a block of oil stocks that are fluctuating rapidly hour by hour. Gary receives the offer an hour before the market closes; he accepts by fax two hours after the market has opened the next morning and after learning that the stock has jumped up significantly. The time period has lapsed if Gary was accepting a fixed price that Karen set, but it may still be open if the price is market price at time of delivery. (Under Section 41 of the Restatement, an offer made by mail is “seasonably accepted if an acceptance is mailed at any time before midnight on the day on which the offer is received.”)
For unilateral contracts, both the common law and the UCC require the offeree to notify the offeror that he has begun to perform the terms of the contract. Without notification, the offeror may, after a reasonable time, treat the offer as having lapsed.