To begin, we need to make a distinction between K-2 petitions on Form I-129F, which are approved by USCIS with four month validity periods, and K-2 visas, which are approved by Department of State consular officers, and are generally issued with 6 month validity periods.
In general, a K-2 visa holder has six months to enter the United States before the K-2 visa expires. If the 4-month validity period of the underlying I-129F petition expires before the K-2 visa is approved, the petition can be revalidated by a consular officer for an additional 4-month period.
To have the petition revalidated, the petitioner must send a written request to the consulate that is issuing the K-2 visa with a statement that the U.S. citizen petitioner and the K-2’s parent still intend to get married.
If a K-2 tries to enter the U.S. after the expiration of the petition’s 4-month validity period, but while the K-2 visa is still valid (during the 6-month period), the K-2 will typically be admitted into the U.S. for 90 days, even if the I-129F petition has expired.
There is no limit on how often a K-2 petition can be revalidated. However, a consular officer may begin to question “the intentions of the couple, especially the intentions of the petitioner in the United States.”
If the consular officer is not convinced that the U.S. citizen petitioner still intends to marry the parent of the K-2 child, the petition will be returned to USCIS by the National Visa Center, or NVC.
Keep in mind that it is not possible to extend a K-2 visa; applications for extensions of nonimmigrant visas are limited to certain categories, and K-2 visas unfortunately are not included. Once the K-2 visa is issued, it must be used to enter the United States before it expires, which is generally within 6 months, and to apply for adjustment of status within 90 days of the K-2’s arrival in the U.S.