The IRS has 3 years to assess additional taxes or penalties, but there are exceptions: if you under-report your income by 25% or more they have 6 years, and if you don't file a return or file a fraudulent tax return there is no limitation - the IRS can assess additional tax and penalties at any time. For state tax fraud the time limit depends on where you live - in CA for example, there is no statute of limitations.
More detail can be found here:
Determining the Statute of Limitations in Federal Tax Cases
