Any time you write an offer, it should have an expiration date for reply as well as an ending date of the contract once accepted. Without that, it is a a VOID contract because it has no end date.
You are free to rescind or withdraw your offer at any time prior to receiving an answer and re-submit it with a deadline, but all must be in writing, due to the Statute of Frauds.
I hope this helps! Also, you may want to consult the advice of an attorney for any legal questions you may have about real estate or contract law.