Generally speaking, you should preserve tax returns and accompanying documentation for a minimum of three years. But:
Three years is the standard IRS guideline (or the fundamental norm in most countries).
6 years: If your income is underreported by more than 25%.
7 years: For documents pertaining to delinquent debts or claimed losses.
Indefinitely: If you either fail to file a return or file one that is fraudulent.
Longer record-keeping is frequently beneficial for future reference, insurance, and loans.