The time between filing the application to the completion is approximately three months. In can take more or less time depending on which INS office you must use and how quickly you complete your paperwork. Some countries require your documents to be notarized and apostilled, which is a state level notary of your local notary. Some counties require require your documents to be notarized and apostilled by your Secretary of State. Other countries require your documents to be notarized and authenticated by your Secretary of State.
Some states require certification with the apostilling or authentication.
Some states do not charge a fee for apostilling documents and others have a per document fee for apostilling. Your agency can help you walk through the paperwork.