From the Stanford web page:
For the 2014–15 admission cycle the Office of Undergraduate Admission will operate the optional alumni interview program (based on applicant’s high school location) in the following areas: Chicago, Raleigh-Durham and Washington, D.C. as well as the following states: Colorado, Connecticut, Delaware, Georgia, Hawaii, Indiana, Maryland, Massachusetts, Minnesota, Nebraska, New Jersey, New York, North Dakota, Oregon, Pennsylvania, Rhode Island, South Dakota, Texas, Virginia, Washington, and Wyoming.
Interviews will also be offered internationally in Austria, Belgium, France, Germany, Hong Kong, Ireland, Japan, Luxemburg, Mexico (Mexico City and Nueva León only), the Netherlands, Singapore, Switzerland, and the United Kingdom. New interview locations will be added over the next several years.
Even if you live in one of those areas, it is possible that you might not get an interview if not enough alumni are available. Interviewers will work on their own schedule, so I don’t think anyone can tell you what the timeframe will be.