Unless you have one of the few illness they define as automatically qualifying
for SSDI, your application will almost certainly be rejected until you appeal
at least two levels (up to the judge).

That process took about 2 years for me. Per their request, I sent
documentation of my illness from about 6 doctors, but SSDI consistently
chose to simply ignore the reports from the 2 doctors who had the
bulk of significant evidence.

To qualify you’ll almost certainly need some objective test results
showing an abnormality. You might be incapacited by sickness, but
unless that can be measured by some significant test, you’re out of
luck. I recognize they use this yardstick to reject fraudulent claims,
but unfortunately it also causes rejection of some qualified applicants.