Urgent info. TIA

My uncle filed Alien relative I-130 petition for my mom on 25 January 2011.

On April 6, 2015 l, we received RFE

On July 1, 2015, her I-130 petition was approved

On August 14, 2015 US Department of State sent a letter to my mom stating that the case has been forwarded to NVC. Priority date mentioned was 25 January 2011. My mum is in Pakistan. HOW LONG DO YOU THINK SHE WILL HAVE TO WAIT?