The general rule is that a revocation is effective when the offeree receives it. For example:
On January 1st, Marsha makes a written offer to Jan to sell her a case of Girl Scout cookies for $50. On January 3rd, Marsha writes Jan another letter revoking the offer. Jan receives the offer in the mail on January 6th and she receives the withdrawal on January 9th. According to the general rule, Marsha’s withdrawal does not become effective until Jan receives it on January 9th.