How far back must an employer go back with a background check, when working in the field of helping people with disabilities?

Must? There is no requirement per se.
What matters is the type of employer, if gov't or private. And gov't agencies may vary, so you need to ask anonymously. Private can go back as far as they are willing to pay for. Most applications show a time limit, such as 'have you been convicted of a crime in the last 5 years.' And some say felony, so that you don't have to mention misdemeanors.
Basically there is no set answer, and you have to tell the truth if there is no time limit. If you are helping people on medication, that puts you in a category that will rule out anyone with a record.