It really depends on the waiting queue in the country from your wife will be apply for US visa, secondly it depends on the class of visa she is applying for. In your case it will be Immigrant visa so it will take 2-3 months if all the documents provided will be up to date. You can also check the duration of processing time of US visa by visiting the following link:
' target='_blank' class='qa'>travel.state.gov
' target='_blank' class='qa'>travel.state.gov
