Thank-you notes for gifts that arrive before the wedding should be sent within two weeks of arrival. Thank-you notes for gifts received after the wedding should be sent within a month of returning from the honeymoon.
It used to be acceptable to take up to a year to send thank-you notes, but that is no longer proper etiquette. To avoid falling behind, it is important to start writing thank-you notes as soon as the gifts start arriving. A guest list should be kept handy to track what arrives and when, and to check off the guest's name when a thank-you card has been sent.