It can take approximately six months to receive a decision on your disabled adult child initial application and another six months on reconsideration.
If your application is denied, it can take an additional year and a half to obtain a decision as your claim works its way through the appeals process. As of early 2017, Social Security has a backlog of one million claims at the hearing level.
While Social Security used to have a focus on paying valid claims at the hearing level sooner, political pressure in recent years has caused it to put a magnifying glass on even the most obvious cases.