I'm a U.S. citizen petitioning for my fiance who lives in Japan. I got the receipt notice for the I-129F visa petition, and am hoping to receive the approval notice soon. Once we get that, how quickly will we need to act? My husband's mother has had some health problems recently, and he may need to delay his departure from Japan (and thus our wedding).
Once you receive approval of your I-129F fiance visa petition, it will be good for four months. (The exact date it expires will be shown on the approval notice that U.S. Citizenship and Immigration Services (USCIS) sends you.)
Within that four-month time period, the National Visa Center (NVC) will communicate with your overseas fiance, asking him to fill out an online form, get a medical exam done, pay fees, and take other steps to get ready for a visa interview at a U.S. embassy or consulate in Japan.
Unlike before, however, he isn't at the mercy of the U.S. consulate when it comes to scheduling. As of 2014, fiance visa applicants must schedule their own visa interviews. So if it helps matters, he could make the appointment fairly late within that four-month window.
If the case gets delayed after that, but your fiance has been actively pursuing it, the consulate has the power to extend the four-month deadline.
Once your fiance gets the visa approved, he doesn't have to enter the United States right away. A K-1 fiance visa is good for up to six months. (Don't make the mistake that some people do and assume that's how long they can spend in the U.S. after entry--a K-1 fiance visa lasts only 90 days, and the holder is expected to get married and apply for a green card during that time.)
For more information on the application process, see the articles on the K-1 Fiance Visas page of Nolo's website.