From a recent report by Social Security's Office of Inspector General (OIG) (emphasis added):
... SSA [Social Security Administration] conducts redeterminations, which are reviews of SSI [Supplemental Security Income] recipients’ non-medical factors of eligibility, such as income, resources, and living arrangements. The redetermination process is a way of detecting any unreported changes in circumstance that would affect a recipient’s SSI eligibility. Redeterminations are scheduled annually if a change in circumstance is likely or once every 6 years if a change in circumstance is unlikely. We identified 53,744 SSI recipients (from 1 of 20 segments) who had not had a redetermination completed in longer than 10 years. We analyzed a random sample of 275 cases. ...
We estimated about 1.1 million SSI recipients — about 1 in 8 recipients — had not had a redetermination completed in longer than 10 years because SSA only did a limited number of redeterminations each year based on its budget. As a result, we estimate d about 77,060 SSI recipients might be overpaid approximately $381.5 million because SSA had not completed a redetermination in longer than 10 years. ...I keep making the same point again and again: Failing to adequately fund SSA costs money. An adequate administrative budget would at least pay for itself by reducing overpayments. Instead, Republicans pursue a wasteful "starve the beast" strategy. It’s ideology over common sense.