This is measured from the day you actually filed your tax return. If you filed your taxes before the deadline, the time is measured from its date, April 15th. Most state tax agencies follow the federal three-year period for auditing tax returns, however, some states have a longer statute of limitations.
Exceptions to the 3-year statute of limitations on assessments and audits:
The IRS has six years from the date a return is filed to audit a tax return and to assess additional tax if the taxpayer omits income that amounts to more than 25% of income that was reported on the tax return.
The IRS also has six years to audit a tax return and assess additional tax on income related to undisclosed foreign financial assets if the omitted income is more than $5,000.
The statute of limitations on audits and assessing additional tax remains open indefinitely if the taxpayer files a false or fraudulent tax return.