Life span of arduino
Atmel chips (the ones used in the Arduino) have been used in all sorts of industrial appilications way before Arduino came along. Same for other micro controllers like the still popular 8051.
What you want to focus on might be the enclosure, seal it against the outside.

Rather than using a (chunky) pre-manufactured Arduino board, you'll have to pick the right chip for your application and design a board around it. http://www.atmel.com/products/microcontrollers/avr/