This article is from the FAQ, by with numerous contributions by others.
Upon approval and successful passing of medical exam and interview at
the foreign consulate, the result will be three distinct deadlines:
(1) A visa will be issued with a validity of 180 days. You have to enter
the United States before the Expiry date shown on that visa
(2) From the date of entry another deadline becomes effective. You
have to marry your fiance(e) within 90 days after entering the USA
on a K-1 visa
(3) After the marriage you have another deadline to demonstrate to INS
that you have married and to apply for change of status. I think,
it is 30 days, but I am not sure. The best is to that immediately
 
Continue to: