At least three years, but six years is preferable. The IRS has three years after you file a tax ­return to complete an audit. The IRS can audit you for up to six years if it suspects that you underreported your income by 25% or more. If the IRS suspects fraud, there is no time limit for an audit, although audits beyond six years are extremely rare.

Probably six years. The government has six years from the date the nonfiled return was due to criminally charge you with failing to file. (There is no time limit, however, for assessing civil penalties for not filing. If you didn't file for 1958, you still have an obligation if you owed taxes for that year.) Not until you actually file a return does the normal audit time limit -- three years -- and ­collection time limit -- ten years -- start to run.

Don't overworry about a nonfiled return due more than six years ago if you haven't heard from the IRS. The IRS usually doesn't go after nonfilers after six years. For more information, see Tax Returns: If You Haven't Filed in a While.

Yes. Filing saves you from the possibility of ­being ­criminally charged or, more likely, from being hit with a fine for failing to file or for filing late. Interest continues to build up ­until you pay. Of course, filing without paying will bring the IRS collector into your life, but he or she will be friendlier if he or she doesn't have to hunt you down. The sooner you start filing, the better.

Probably not. Although you can get extensions to file your tax return, you still must pay by April 15 or the IRS can impose a penalty and charge interest. You can try pleading hardship on IRS Form 1127 to get up to six months ­extra to pay, but the IRS may require that you post a bond or mortgage, which is impractical for most people. Few payment extensions are granted. Even then, only penalties, not interest, stop accruing. Form 1127 works best in requesting an extension to pay ­estate taxes. For more information, see Filing and Paying Taxes Late.

Maybe -- the idea is frequently kicked around in Congress. However, the IRS has always opposed tax amnesty legislation, which lets nonfilers come forward without being criminally ­prosecuted or civilly fined. The IRS's reasoning is that after the amnesty period expires, significant numbers of people won't file, expecting another ­amnesty. Based on the success of various states trying, the IRS may be wrong.

Federal law makes IRS files private records. The law has many exceptions, however. IRS files can be legally shared with other federal and state agencies. (Most leakage comes from result of sloppy state agencies that are granted access to IRS files.) Furthermore, IRS ­employees have been caught snooping, and computer hackers have broken into government databases. While violation of the Privacy Act is a crime, rarely is anyone prosecuted for it, though IRS personnel can be fired if caught.

Yes, but you don't get the reward until the IRS ­collects from the cheater, which is far from a sure thing. The IRS pays about 8% of the first $100,000 it collects and 1% of the balance. ­Identities of informers are kept secret, but tax cheats usually know who reported them -- mostly ex-spouses or disgruntled business associates.

In a recent year, the IRS paid out a paltry $1.5 ­million for tips, on $72 million collected. The IRS places low priority on investigating tips and paying rewards. Typically, you will never know what action, if any, is taken on your tip, but if you want to try it, submit IRS Form 211.

If you filed your tax return at least eight weeks ago, call the IRS tax refund hotline at 800-829-4477, Monday - Friday, 7:00 a.m. to 11:30 p.m. Or, call the 24-hour ­assistance number at 800-829-1040 and ­request assistance from the taxpayer advocate.

If you filed your return on or before April 15 and don't receive your refund until after May 31, the IRS must pay you interest.

If you never get a refund, it may have been intercepted to pay any of the following:

• state or federal taxes you owe
• a defaulted student, SBA, or other federal government loan
• delinquent child support
• a public benefit overpayment (such as HUD, VA, or Social Security).

In these ­situations, you are supposed to be notified in writing, but don't count on it.

It depends. The Internal Revenue Manual states that "taxpayers should not be held liable for interest on … erroneous refunds if the IRS was clearly at fault … and the taxpayer is cooperative in repaying." However, if you caused the erroneous refund and now can't repay it, the IRS can and will charge interest.