Maybe -- amnesty periods, which let nonfilers come forward without being criminally prosecuted or civilly fined, are discussed in Congress most years.
But the IRS has always opposed tax amnesty legislation. The IRS's reasoning is that, after the amnesty period expires, significant numbers of people won't file, expecting the IRS to have another amnesty program. Based on the success of the states that are trying amnesty programs, however, many tax professionals think the IRS is wrong.
It can be a good idea to file old returns, at least from the past six years. If you voluntarily file your old tax returns before the IRS notifies you that you are under criminal investigation, the IRS will usually not prosecute you criminally for your original failure to file those tax returns. For more information, see Stand Up to the IRS, by attorney Frederick W. Daily. It includes a chapter on what to do if you haven't filed, as well as a chapter on tax fraud and crimes.
At least six years, and possibly forever. While the government has only six years from the date the nonfiled return was due to criminally charge you with failing to file a tax return, there is no time limit for collecting taxes and assessing financial penalties for not filing. It is not until you actually do file a return that the audit time limit -- three years -- and collection time limit -- ten years -- starts to run.
As a practical matter, however, if you haven't heard from the IRS in six years, you don't need to worry too much about taxes owed on a nonfiled return. The IRS usually doesn't go after nonfilers after six years -- unless the IRS begins its investigation before the six years elapsed and you owe a large amount of taxes. After six years, the IRS frequently purges its computer files.