You can check current processing times in USCIS website:
Select for I-824 and processing center.
normally it takes 3-6 months to get duplicate copy, but in my case it took too much time.
My employer applied for duplicate copy in Nov-2017, and I received a week ago on 25-Jan-2019.
my I-824 was approved in Dec 2017, but USCIS didn't send the copy because of some problem with their system.
After many followups I received.
if your application date is beyond current processing time then you can raise a ""
Outside normal Processing time" with USCIS to check ur statu.
FYI, I traveled to US with out I-824, fortunately no one asked me for approval copy at immigration.