How far back can the US go back for a audit?

I believe the current policy is SIX YEARS without managerial approval. However, if you failed to file a tax return from anytime in the past and the IRS determines that you had sufficient income to have a filing requirement then Congress requires the IRS to file what is known as a Substitute For Return for you. The SFR will list ALL the income that the IRS is aware of.

If, in previous years, you filed as Married Filing Jointly then the IRS will allow the Married Filing Separately standard deduction. If not, then the IRS will allow the Single standard deduction. Either way you will be allowed YOUR OWN exemption only - even if the previous year return had dependents listed.

The IRS can get somewhat "tedious" with back-due taxes. You should acquire the services of a Tax Professional to "represent you" before the IRS instead of you having to go to see them.