221(g) white form F1visa

I have done with my F1 visa interview on December 08, 2017. I was given white form 221(g). Its been 15 days and still they didn’t update any thing. When will the update ? How long did they process ?

My passport was returned to me on the day of interview.

They is no stipulated time. It depends on case by case basis. You just need to wait, there is nothing else you can do about it. It make take weeks or even months at times.