Q:

Arduino Lifetime

I have made an RPM datalogger using Arduino that stores RPM-data on an SD card. I have intefaced LCD with arduino (used 6 digital pins of arduino), an RTC (DS3231,used 2 analog pins), SD card slot(4 digital pins) and an Reflective sensor(1 digital pin). Thus, using a total of 11 digital pins, 2 analog pins, Vcc , 3.3V and Gnd pin.I have applied 12V to arduino through barrel pin. I have put my module in an industry at temperature around 40C.
I want to know the lifetime of Arduino. 
Till how many years it can work safely ?  

A:

The parts most likely to fail first on an electronic device are:

Electromechanical parts.
In an official Arduino, these are the connectors and possibly a pushbutton. Some Arduinos have slide switches and such as well.
I assume you aren't going to press the reset button very often, so we can ignore it.
If there are any other buttons or switches — e.g. the settings slide switches on a Seeeduino — I assume they're going to be set and left there for the rest of the device's usage life, so they can also be ignored.
That leaves the connectors.
Vibration is a problem in industrial applications. If you let the DC power plug get vibrated out every month, the vibration and repeated re-plugging might put enough wear on the connector to make it unreliable. This won't kill the Arduino, but you can expect intermittent reboots, which many applications won't tolerate. I recommend hot-gluing the barrel connector in place, and strapping the DC power cable down to something solid nearby. 
Be sure to leave enough slack in the power cable that vibration and tugs on the cable don't translate to angled tugs on the connector, stressing the solder joints.
Shields have enough contact area that they shouldn't come loose on their own, unless there's flexion involved.
Ethernet and DB-9 serial connectors are fairly tolerant of vibration, but you should still strap the cables down to prevent their weight from dragging on the connector.
Electrolytic capacitors.
Some Arduinos have these, while others get by with lower-valued solid tantalums. Tants are more reliable, particularly at high temperatures.
As a rule of thumb, electrolytic capacitor life doubles with every 10°C drop in temperature. A really nice electrolytic cap will be rated for 5,000 hours of life at 105°C. 45°C is six such doublings, so 64 × 5,000 hours, giving an expected lifetime of ~36 years.
Sounds fine, right? Now consider that a cheap cap might only be rated for 1,000 hours at 85°C. That's only 4 doublings, or 16 × 1,000 hours, which is only about two years of continuous use.
The story is more nuanced than this. By "lifetime," I don't mean that electrolytic capacitors magically cease to be capacitors at the next hour past their rating. Instead, what happens is that the manufacturer has made an educated guess that the capacitor will still be within its rated limits after that many hours of use.
After the rated lifetime, the capacitor is still going to be a capacitor, but its accumulated drift is allowed to take it out-of-spec once it's given you its rated service life. The more well-made sort of Arduino board will have some design margin that can absorb some spec drift. So, even if you get only exactly as many hours of life out of the cap as the manufacturer guessed you would, the Arduino might continue working.
Anything connected directly to a raw power source.
It doesn't take a very big power surge to kill a delicate microcontroller. To some extent, the fact that you're using the on-board regulator helps. But, consider adding some outboard protection components: zener clamps, maybe a polyfuse, etc.

A:

The design of the Arduino has changed over the years and there are many other companies making Arduino-compatible devices. Some of these will be cheaply made and have a short lifetime. Some will be especially well-made (for example the Ruggeduino) and may last much longer.
Since the various Arduinos are primarily experimenters prototyping boards, I doubt anyone has performed and published accellerated MTBF testing.
I believe the first Arduinos were made in 2005 and there have not been (so far as I know) any reports of unexpectedly high levels of failures.

Serial Arduino 2005 - image by Nicholas Zambetti 
I guess most Arduinos will fail eventually due to misuse, overheating and (maybe) eventual failure of electrolytic caps.

P.S. 1 data point: I'm still using a clock-radio manufactured in 1983 and used continuously 24/7/365 since then. I know of no reason why an Arduino could not last that long.

