I used to keep mine up to five years but then found out that was too much. It’s three years back per the IRS. After three years you cannot be audited and you will hever be questioned beyond three years prior. So pitch it!
It's 6 years if the IRS believes you significantly understated your income or overstated your basis in the sale of a capital asset. If your income is well documented on W-2s or 1099s and you don't own any foreign property or bank accounts, then 3 years is fine.
If you sell a high-value house and are adding a lot of improvement costs made over several years to the basis, then it's probably a good idea to hang onto that documentation for 6 years.