Do you know how long will it take to get a US visa?

US visa is one of the most difficult visas to obtain, for a young person.  The time it will take depends on the type of visa you will be applying for and your qualifications.

From what I heard: immediately after the interview. Short answer: No

what is your purpose?

if it's tourism purpose

u just need to apply by web then you get one, since you are korean..

We're are trying to get a fiance visa for about 3-1/2 years now.  We were turned down on the first application, which apparently is a common occurrence.  The application is supposed to take 5 months in processing here in the states and then sent to the local embassy where the interview is arranged.  So far it is in it's seventh month here in the US.

what kind of visa?if  visa for studying is very difficult to get. you need long time to demonstrate your expense

If you are a Korean passport holder, you are eligible for the VISA WAIVER PROGRAM (VWP)

You get their answer through email a few minutes after you apply and pay. More info below.

Visa Waiver visitors must apply for travel pre-clearance at least 72
hours prior to departure at

https://esta.cbp.dhs.gov.

Travelers must pay a fee of US $14.00 by credit card to apply for
travel pre-clearance. The website takes the name, birth date and
passport information of the intending Visa Waiver visitor and
immediately provides one of three results: authorization approved;
travel is not authorized; or authorization pending (if more time is
needed to make a final determination). Travel authorization remains
valid for two years or until the Visa Waiver visitor's passport
expires, whichever is first.

This isn't the right site for your question.  Try this one instead: Visa Journey 

I've seen people wait over 5 years, that had a perfectly legitimate relationship, then others go in that I thought were unlikely to get approved...get approved! There's really no way of knowing how long it will take. 

If it gets too tough send me a message and I can recommend the best US visa lawyer in Vietnam - Marc Ellis.