Federal law requires you to maintain copies of your tax returns and supporting documents for three years. This is called the "three-year law" and leads many people to believe they're safe provided they retain their documents for this period of time.

With more and more United States citizens earning money from foreign sources, the IRS reminds people that they must report all such income on their tax return, unless it is exempt under federal law. U.S. citizens are taxed on their worldwide income.
Contrary to popular belief, there’s nothing in the U.S. Constitution or federal law that prohibits multiple states from collecting tax on the same income. Although many states provide tax credits to prevent double taxation, those credits are sometimes unavailable.