For the actual tax return itself, the IRS advises keeping them forever. I would 100% agree with that. Especially in the digital age, where everything can be saved on a hard drive or backed up in the cloud, there's no reason to toss your actual tax returns.
According to the IRS, you should keep records relating to property “until the period of limitations expires for the year in which you dispose of the property.” These documents would include anything that supports your calculations for depreciation, amortization, or depletion deductions.
For example, if you bought a house 20 years ago and sold it this year, you would need to keep any records related to that property that supports the deductions claimed on your tax return for an additional three years.