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In my experience (NOT from Islamabad), it takes about three months.
German legislation indicates that an application must be processed within three months and, if not, you can sue the authority for faster processing - unless the rasons for the delay are outside of their sphere of influence.
I am not sure if this (sueing in a German court) is practical from where you are, but in any case you should contact the embassy and ask about the status and what is holding it up.

Currently I am in Berlin but family is waiting for visa in Islamabad. I tried contacting embassy in islamabad via phone, email, mail and fax but no reply on any channel. On contacting Auslanderbehorde they tell me your certificates verification documents are not available. However verification guy visited my home in Feb 2018. Its been 5.5 months since his visit and exactly 6 months and 8 days since submission of application. Do you think sueing in Immigration Court Berlin will do any help?

As far as I know, sueing for inactivity is not done in an immigration court but in an administration court. But please consult a good lawyer first- he/she might know of other methods.