There is no way to know for sure how long it will take, but there are a few things that you could do to move it along:
1) Put in a service request for the case being over processing times - select "case outside normal processing times" at https://egov.uscis.gov/e-request/Intro.do with your husband's I-485 receipt information and explain that the interview was in January 2016
Please note that doing this is not likely to get the case adjudicated, but you must do it to show that you are trying to exhaust your administrative remedies
2) Contact your Congressperson's office (find representative at https://www.house.gov/representatives/find/) and put in an inquiry through the Congressperson
3) After trying the above two and at least getting case numbers for both, you can put in a case assistance request with the USCIS Ombudsman (https://www.dhs.gov/case-assistance) - note that they like to know that you have already tried other things, hence first try the first two.
You will need to wait up to 45 business days after trying #3, but it is often effective.
If it is not, the last resort is
4) File a writ of mandamus - you'll need a lawyer for this. It's basically a lawsuit against the government, compelling them to adjudicate the case.
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