Solid state electronics fail pretty-much randomly, but average life is very-very long as long as it's not electrically, mechanically, or environmentally abused.
This is from [u]Atmel[/u] and it only applies to the Amel ATmega chip. The more components you add, the higher the chance of a failure and your MTBF is reduced:
Mean Time Between Failures is an indication of the number of hours to pass between failures.
Please find below the predicted MTBF (Mean Time Between Failure) numbers of
our micro's at different temperatures. The statistical calculations are based
on current reliability qualification data in the "microcontroller
reliability data package".Atmel has been shipping NVM parts for 20 years and Flash Micro's for 13
years. We have never experienced a long term reliability problem.Here are the MTBF numbers calculated from life test and data retention
results:65ºC 1.69x10e7 hours. => 1929 years
85ºC 4.46x10e6 hours. => 509 years
105ºC 1.34x10e6 hours. => 153 years