Is it possible for the extended life to go beyond the legal life (20 years) of a Patent?

No - there is no extension of time available beyond the statutory limits. For US patents filed on June 8, 1995 or later the term is 20 years from date of filing. For patents filed prior to that the term is either 20 years from date of filing or 17 years from date of issue of the patent, whichever is longer - so for example it's possible today for a patent to still be in force that was filed in 1993 but not granted until 1998. The patent remains in force for this period as long as the patent holder continues to pay the required recurring fees to the US Patent and Trademark Office.