Hi Anita,
As per my experience it purely depends on case to case to basis. In my case CO raised a silly objection which you may read in this thread and I had provided evidence to counter that objection so my case was decided with in 29 days after NJL. But I have read and heard from migration agents that it can take longer if issue is complicated.
Some people also decide to withdraw application after NJL, because if it is with PIC 4020 condition, you may get banned for applying any visa for Australia for next 3 years. If you are confident that no wrong information is provided and nothing is bogus, it is good to respond to NJL very thoroughly with all the possible evidences which may clarify any doubts Home Affair may have due to which NJL is being issued and than wait for CO response without bugging them. If not, than withdraw your application to avoid ban.
Regards
Anfal