Well you haven't really given a lot of details. I am assuming you're going for a visa. However, you must realize that if you have ANY problems of inadmissibility, i.e., an illegal entry, past criminal record, or unauthorized time in the US, you will have problems at the consulate and may be there up to a year or more. Before you leave the United States, you should contact an immigration attorney if any of these describe you.
If you have no problems in the past and do not expect any at the consulate, I would check the current processing times at the DOS website for Ciudad Juarez. There you should be able to find out exactly how long it is taking for your visa to be processed.
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However long it takes for the Consulate to issue you the visa.
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Before you leave, I really recommend checking with an attorney to see if you are admissible, and if not, if there is a waiver available for you. A lot of people make the mistake of leaving for their interviews when they are never going to get a visa to come back in. I really think it is worth it to check first.
The amount of time you will be outside the country will depend on whether or not you need a waiver. But you should also know that the embassy in Ciudad Juarez is all over the place right now and no one can really predict how long it will take. I would count on spending at least a few weeks there, just to be prepared, and months if you need a waiver.