The validity period of an immigrant and fiancé(e) visa is based on the medical examination, the maximum validity of which is six months from the date on which it is performed. You will be advised further at your interview.
K-3 visas are usually valid for travel for two years from the date of issuance. A K-4 visa is valid for two years or until the day before the holder’s twenty-first birthday, whichever is shorter.
If you are unable to travel to the United States before the visa expires, you should return the visa to the Immigrant Visa Unit with a covering letter explaining why you have been unable to travel. Please do not return the visa until it has expired.
Your case will be reviewed by a consular officer and if you are able to show, to the satisfaction of the consular officer, that the reason for not traveling was due to circumstances beyond your control, a new visa will be issued. You will be required to pay new visa processing and issuance fees and may be required to attend a further medical examination. Further information will be provided to you once your case has been reviewed.