How long will program stay in memory without power?
More than 20 years, and probably more than 100 years.

That's the guaranteed minimum:


  Data retention: 20 years at 85°C/100 years at 25°C(1)
  
  (1)  Reliability Qualification results show that the projected data retention failure rate is much less than 1 PPM over
  20 years at 85°C or 100 years at 25°C.