In absence of those parameters, I would take the Flash data retention time as first approximation.
And if the application includes IAP, the estimated number of erase/write cycles vs. the guaranteed number.
Under average load and little deviation from moderate temperatures, an average MCU hardly fails for electrical reasons.
Elevated temperature, e.g. like those defined for automotive environments, will decrease the lifespan (both electrically, and Flash code fails).
Increased elevation (i.e. increased cosmic radiation) will result in more pseudo fails (Flash/RAM corruption).