As you can see by the answers, Lymer, the best answer is, "It depends."
First, it depends on the type of contract you are using. For a standard residential purchase, (not a foreclosure or short sale) there will be an expiration clause at the end of the contract. In California, you can specify a date, or, if none is specified, the contract defers to the default language. The default language give three days (calendar, not business) for acceptance.
If the contract is a short sale purchase, there is an additional addendum which specifies a date under which the contract becomes voidable if the lender (not the seller) has not approved the short sale. This date is important, because it allows for the return of your deposit if no approval is forthcoming. (It also gives the seller the right to cancel, so be sure you discuss the date with your agent.) YOUR STRATEGY MATTERS!
Each lender uses a different purchase agreement/ addendum for their REO (foreclosure) sales. They are created by counsel for the bank/ investor, and are usually not modifiable. Pay strict attention to all dates, and the rights they mitigate.
I respectfully disagree with agents below who have said not to put in an expiration date. Your rights under the contract are limited by dates, but SO ARE THE SELLER'S RIGHTS. An open ended contract (no expiration) gives the seller the opportunity to take your offer and hold it, without a response, while they see what else may come along. Of course you can rescind the offer, but wouldn't you rather have an up/ down decision from the seller in a timely manner? If your offer expires, you can always extend it in writing (California has a form specifically for that purpose) or resubmit it.
More to the point, and most important, your agent is DUTY BOUND under the responsibilities of his agency relationship with you to go through, line by line, the entire purchase contract. He should explain the implications of each paragraph, and ASK YOU what you want as an expiration date (as well as every other term of the contract.) If he simply "forgot" that line, then he should rectify that now by submitting a new contract with an expiration date, if you desire. (And what else did he forget, or neglect to discus?) At the very least, he should be prepared to tell you why it was not discussed, and what that will mean for you in your transaction. If he cannot, then you need a better, more thorough Buyer Agent (preferably an Accredited Buyer Agent [ABR]) to represent your interests.
The Bremner Group at Coldwell Banker
REALTOR, 00588885, ABR, CDPE, eAgent, CSP, SFR, HRC, CRE
(O) 310-571-1364 DIRECT