There are various limitations periods such as one that applies to assessments of tax by the IRS and refunds, another that applies to collections by the IRS, another that applies to large understatements of income, etc.
See https://www.americanbar.org/groups/business_law/publications/blt/2017/08/06_wood/
There is no specific limitation on how far back a taxpayer can file an original return or an amended return. Refunds, however, will not be paid after the limitations period expires.
