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.