How far back can the IRS audit a business?

Generally, the statute of limitations on assessment of a tax deficiency is three years from the date a tax return was due UNLESS the deficiency was substantial, meaning a return failed to include 25% or more of the gross income it should have, in which case the statute of limitations extends to six years. And there's no statute of limitations on a taxpayer who was required to file a return and failed to do so.