If the K1 beneficiary is qualified to receive this visa, s/he may be admitted to the United States for the purpose of concluding a marriage to the petitioner within a 90-day period.  Please note that the K1 visa is actually a single-entry non-immigrant visa; for a beneficiary to derive immigration benefits s/he must marry within 90 days of entering the U.S. on a K1 visa and apply with USCIS to adjust to CR1 status.  If the beneficiary leaves the U.S. without adjusting to CR1 status, s/he will not be allowed to re-enter the U.S. without first applying for and receiving a new visa.