Social Security Disability "Review"

My husband was approved for disability in 2011 for a bone-on-bone knee. He had the knee replaced in 2014. It's better, but there's no way he could go back to work in the heavy construction industry. He's been working part-time at a grocery store. He's 63.

We filled out the review papers about 3 months ago, but haven't heard anything back from them, and phone calls to the local office where we sent the papers are ignored. Anyone know how long a review takes? Thanks