How long must you worry about the risk of being audited by the IRS?

No one wants to be audited by the IRS, and it is only natural to worry about it. Even if you think your tax return is pristine, gathering receipts is no fun. Explaining what you did and why isn't either. Of course, if your returns have unusual or aggressive items that you may be especially nervous about, the waiting and the worry can be much worse.

So how long must you worry? It pays to know how far back you can be audited. You can influence the answer, and there are steps you can take to cut your risk. First, let's start with the basics. The IRS usually has three years after you file to audit you. But there are many exceptions that give the IRS six years or longer.

The three years is doubled to six if you omitted more than 25 percent of your income. For years, there was a debate over what it means to omit income from your return. Taxpayers and some courts said "omit" means leave off, as in don't report. But the IRS said it was much broader, including reporting that has the effect of an omission of income.

Say you sell a piece of property for $3 million, claiming that your basis (what you invested in the property) was $1.5 million. In fact, your basis was only $500,000. The effect of your basis overstatement was that you paid tax on only $1.5 million of gain, when you should have paid tax on $2.5 million.

In U.S. v. Home Concrete & Supply, LLC, 132 S. Ct. 1836 (2012), the Supreme Court rejected the IRS argument, and held that overstating your basis is not the same as omitting income. The Supreme Court said that three years was plenty for the IRS to audit. Then, Congress overruled the Supreme Court, and gave the IRS six years in such a case.

Six years is a long time. Filing your tax return early won't help either, since the three or six years runs from the later of your filing date, or the due date. The time periods can be even longer in some cases. For example, the IRS has no time limit if you never file a return or file fraudulently. Even so, the practical limit for the IRS to go back is usually six years.

Another scary rule is that the IRS has an unlimited time to audit if you omit certain tax forms. An example is IRS Form 5471, a reporting form for interests in a foreign corporation. If you omit it, the statute of limitations on your entire return never even starts to run.

There are special statutes, too. For example, once a tax assessment is made, the IRS collection statute is typically 10 years. And, in some cases that 10 years can essentially be renewed. That's one reason the IRS can sometimes go back decades. In Beeler v. Commissioner, T.C. Memo. 2013-130, the Tax Court held Mr. Beeler responsible for 30-year-old payroll tax penalties.

Figuring out the applicable statute of limitations that applies to your situation can be complex, and then waiting it out can be nerve-wracking. An audit can involve targeted questions and requests of proof of particular items only. Alternatively, audits can cover the waterfront, asking for proof of virtually every line item.

Audits can drag on too. Frequently, the IRS says it needs more time to audit. The IRS will ask you to sign a form extending the statute of limitations, usually for a year. If you don't sign, the IRS will send you a tax bill, usually based on harsh assumptions. Most tax advisers generally tell clients to agree to the extension. However, it's best to get some professional advice about your own situation. You may be able to limit the time or scope of the extension.

Another hot button that impacts the statute of limitations involves offshore accounts. The IRS is still after offshore income and assets in a big way, and it dovetails with another IRS audit rule. The IRS also gets six years to audit if you omitted more than $5,000 of foreign income (say, interest on an overseas account). That matches the audit period for FBARs, annual offshore bank account reports that can carry civil and even criminal penalties far worse than those for tax evasion.

For all these reasons, be careful and keep good records. After a time -- many people say seven years -- you should be able to throw out records and receipts. However, keep copies of your old tax returns forever. Plus, some records such as improvements to property that go into your basis should be kept too. If you remodel your kitchen and sell your house 20 years later, the receipts for your remodeling job are still relevant to your tax return.

To help lower your risks, keep good records, and keep copies of all your past tax returns. Report all your income, and disclose your tax positions on your return, even if you are claiming the money isn't taxable, is taxable as capital gain, or whatever. Also, consider having your return professionally prepared.

Report offshore accounts on both tax returns and FBARs, and make sure you file any other forms that could extend your statute to six years or forever. Steer clear of tax shelters and things the IRS counts as 'listed transactions' that can bring trouble. If you have big tax issues about a particular issue -- say a lawsuit recovery, casualty loss, etc., consider getting targeted tax advice for that item from a tax lawyer or CPA.

Finally, if the IRS does contact you, consider getting professional advice. And don't ignore the IRS. Sometimes, issues can be addressed easily if you do it carefully and timely.