microcontrollers are actually designed to work continously executing instructions till power down. the manufacturers would specify the normal and maximum working voltages, current, ambient heat, stress, vibration etc. when these specifications are
taken care during designing the applications using micro will work as long as any other components whose life span is shorter like cap, regulator, resistor breaks down. no semiconductor manufacturer can guarrentee you that his micro can work for this much years continuously.
Better use a wake on interrupt mode and put hte micro to sleep, this way your micro may be made to consume less power.