How long will it take to cancel an American Visa?

My finace got her k1 visa in March,but then we started having many problems. I sent a K1 finace visa cancellation letter signed and notarized about two weeks ago. I sent one letter to the place I officially applied to in California and the other one to the home country emby which is in Bogota, Colombia. When or will I get a letter stating the official cancellation. Need to know so that my x won't use her visa to get in the United States.