The IRS has a window of 10 years to collect back taxes from you. After that period, they are compelled by law to cease from conducting any collection activities against you. This window begins when one of two criteria is met: when the IRS officially determines that you owe taxes, or when the result of an IRS audit is finalized.
Be careful, though, because you could unwittingly extend this 10-year window indefinitely. For instance, filing an offer in compromise, collection due process appeal, innocent spouse request, and even bankruptcy permits the IRS to collect money from you beyond the 10-year limit.
Once the statute of limitation has expired, the IRS will have to make modifications to your tax records and credit your account for the amount of unpaid taxes, penalties, and interest. You can obtain transcripts from the IRS that verify you no longer owe them.
If you want to determine when your statute of limitations started and how much time the IRS has left to collect from you, consult a tax attorney.