What is the statue of limitations for tax evasion ?

The statute of limitations of a crime is the amount of time a prosecutor or a plaintiff has to file charges. In the case of taxes, it represents how long you should be looking over your shoulder after – willfully or otherwise – lying on your tax return.
The general rule of thumb is that the IRS has 3 years to audit your tax returns. If an investigation of your tax return reveals you concealed over 25% of your income, the IRS gets twice the time, 6 years, to file charges. However, this time period can be extended for a variety of reasons.

No Statute of Limitations: if there is a "willful attempt to evade or defeat tax", there is no statute of limitations for assessment. The IRS can audit your return at anytime if they can prove fraud or evasion. Another matter to consider is when the 6-year period starts. The IRS could prosecute a series of fraudulent tax returns (especially outside the 3 year lookback period of the taxpayers QAR of his QD submission) as a single charge and only start counting the 6-year period from your last act of tax evasion or fraud. Although the IRS is limited to how far back it can look when filing charges in criminal court, there is no SOL for civil tax fraud. This means the IRS can look back as far as it wants when suing for civil fraud. In practice the IRS rarely goes back more than 6 years because it has a high enough burden of proof to meet in fraud cases without having to deal with the added difficulties of proving older charges.
The 10 year statute of limitations is for the collection of tax. Once tax has been assessed (either through the filing of a return or through an audit of a return within the assessment statute of limitation timeframes the IRS has 10 years to collect the tax from you.
Now in terms of doing a QD,  I usually start my analysis -- start, not end -- with the applicable civil statute of limitations absent fraud. Absent fraud or failure to file some foreign information forms (3520, 3520-A, 5471, 8938) that keeps the statute open, the statute will be either 6 years or 3 years. If, outside the offshore bank context, fraudulent returns or FTF may be involved, I consider the criminal statute of limitations -- 6 years. Most practitioners, treat the 6 year criminal statute as the maximum number of years that should be filed in a QD. Of course, with an open civil statute for earlier years, the IRS can audit those years and make adjustments. I think most practitioners experiences are that the IRS rarely, if ever digs back into earlier years, but that is a risk that cannot really be assessed in advance.
 So, considering the two above, the next point in the analysis is to assume that a maximum of 6 years amended or delinquent returns will be filed. Taxpayers with little risk tolerance will file 6 years. Taxpayers with more risk tolerance may drop off 1, 2 or even 3 of the earlier years -- so that only 5, 4 or 3 years amended or delinquent returns are filed.
By filing amended returns (1040X), no accuracy related penalties will apply because it is a qualified amended return (QAR). If the IRS were to do the work necessary to develop a civil fraud case, of course, the civil fraud penalty could apply, but I have not seen that happen and I think most practitioners have not seen that happen outside the offshore context. Now, of course, the IRS has noised that the offshore context may be different, so that it might assert fraud on QD's.
One thing I think you may infer is that the filing of a QD would be a new willful fact independent of earlier acts. I don't think so, if the QD returns and FBARs are fair and correct. It does not solve the original fraud (as the Supreme Court famously held in Badaracco in the statute of limitations context), but it is not a separate criminal act nor would a QD permit an inference of a criminal act.
 Of course, the additional tax underreported originally is an element of the crime of evasion, but the IRS would have to prove that by clear and convincing evidence, a difficult burden to meet. Indeed, as I recently posted, it is the taxpayer who will want to establish an inference of non willfulness by filing amended returns so that the jury hopefully will be persuaded that, when he learned he had done something wrong, he corrected it.


http://www.irs.gov/Businesses/...
".10 Sections 1.6661-6(c)(2) and 1.6664-2(c)(3) generally provide that for purposes of the substantial understatement penalty and the accuracy-related penalty, respectively, a "qualified amended return" is an amended return that is filed after the due date of the return for the taxable year and before the time the taxpayer is first contacted regarding an examination of the return. A qualified amended return also includes an amended return that is filed within such time frame solely to disclose information but does not report any additional tax liability. In addition, sections 1.6661-6(c)(4) and 1.6664-2(c)(4) provide that the Commissioner may prescribe by revenue procedure the manner in which the rules governing qualified amended returns apply to particular classes of taxpayers."
"In the case of additional tax shown or adequate disclosure made on a "qualified amended return," however, section 1.6661-6(c) provides for an automatic waiver of any penalty that would not have been imposed if the additional tax shown or the adequate disclosure made had been included on the taxpayer's original return. Section 1.6661-4 describes what constitutes adequate disclosure."

http://legal-dictionary.thefreedictionary.com/Statute+of+Limitations
http://federaltaxcrimes.blogspot.ch/2014/07/convicted-politician-did-not-lay-proper.html