I 130 was approved and sent to NVC May 2015

I am from Pakistan and would like to know that my sister who is US citizen filed I 130 for me in April 2011 and I got approval from USCIS on May 2015 and they sent my case to NVC in May 2015 as well. After that I and my sister both received NVC welcome letter August 2015 since than there is no contact yet from NVC. I would like to know how much time will it take more at NVC. Thanks in advance for your assistance and support