Generally, an applicant is required to appeal within 60 days (+5 days for mailing) from the date of the denial letter. However, SSA considers "Good Cause" for late filing. They will often accept the appeal request late for someone who is experiencing homelessness or has a serious mental illness. They are also accepting Good Cause for late filing due to COVID-19. We definitely recommend appealing the decision (to preserve the original protective filing date and potential back-payments).
Here is a link to a sample Good Cause letter: https://soarworks.prainc.com/sites/soarworks.prainc.com/files/Sample_Letter_to_SSA_Good_Cause%2BCOVID19.docx