We're beggining a project which could be for 2 years or more.
What about the life time of an arduino in years ? Has something a feedback on this kind of projects ?
Cheers
Sebastien
Well I've never done a project like this, but if used correctly I think an Arduino can last for 2 years.
I have Arduinos that are 2 years old and still work fine, but these weren't used continuously.
Should be no different than any other modern electronic device, assuming it is used within specifications. I would expect far more than two years. Since it has relatively few components, and therefore fewer parts to fail, I might even expect an Arduino to outlast common consumer electronics like phones, TVs, cameras, etc.
iv got one thats been going for 5 months now its still running fine. And at work we have a couple that have been constantly running for a year and a half. So as long as your not doing anything horrible to it, 2 years should be fine.
Solid state electronics don't "wear out". Failures are very infrequent and basically random (assuming no electrical, mechanical, or thermal abuse). On average, 2 years should be no problem. But if the design is critical, such as life-support, or if it's going into space where it can't be repaired/replaced, you have to take extra precaustions, because failures are random and there is a slim possibility that you get a failure after 2 days, or 2 weeks...
Take a look at [u]this[/u]. (That's just for the chip. Of course, there are more parts on the Arduino board, and you might be adding some parts. If you use 100 parts, life-expectancy (MTBF) is 1/100th.)
I do have an Arduino that "went bad" after a couple of weeks... The chip is running, but the bootloader doesn't work. (I don't know if the bootloader somehow got corrupted, or if the serial-chip "went bad" or somehow got burned-out.)
I built a few projects with a different microcontroller about 15 years ago, and they still work fine. One is a car alarm that's been running 24/7 for more than 15 years... even when it's not armed. The only time the microcontroler & software-loop haven't been running is when the battery was dead!
Mechanical parts (switches, relays, motors) and connectors can wear-out and connections can become corroded. Electrolytic capacitors can have lifetimes less than most other "electronics". And of course, vacuum tubes age, deteriorate, and wear-out. 
Also, the flash data retention is quoted 100 years at room temp, or 20 years at ridiculously high temp, IIRC. So, 2 years (assuming it's not in a pot of boiling water) should be cake.
For long-term, unattended operation, I would encourage the use of a watchdog timer to reset the chip if the code crashes or locks up for whatever reason. This is built-in to the AVR, and there are examples in the playground.
Data point:
I have 14 '328 based boxes (mix of promini's and standalone designs) that have been powered up 3-4 times a week and run for 2-5 hours each time, have been doing that since Dec 2010. 1 promini failure - the box it was connected to via serial interface also died, no way to say which one caused the other to go.
As pointed out electronics can suffer from a random failure at any time.
I did build a ham radio repeater controller that worked for 17 years 24/7 before they turned if off when they moved site.
The most likely to fail part is the power supply, cheap ones have capacitors that have a short life. Large electrolytic capacitors unlike most other bits of electronics do have a wear out mechanism and the cheaper ones will last about 2 years, so don't economies on the power supply.
You are going to find a shorter bathtub curve in components like power supplies and so on. It is usually passive components like capacitors that fail before semiconductors. Keep things cool, and over-design so things are well within spec.
(And there we have it. I should have read all the way to the bottom of the thread.)
Great answers,
I'm confident, now.
a thing worth mensioning imo is that when u use timers beware of the overflow i've red that for millis() this happens once in about 50 days and for micros() it happens once in about 70 minutes