Will using my iPad as an external monitor negatively affect it's lifespan?
With regard to your question about a realistic lifespan:

I would define the life expectancy of any device as however long its user can use it for its intended purpose. Which will cease to be the case when something physically breaks or when functionality is phased out (e.g. AirDisplay functionality is removed in a future update). It is impossible to predict if and when either of these things will happen to your iPad in the next few years. There are simply too many factors involved.

In my own experience, we have a 1st gen iPad (bought end of 2010) in the office that is on 24/7 (no screen timeout) as an interactive wall calendar and it is still going strong. On the other hand, one of my coworkers bought a 3rd gen iPad a few months ago and it died after a few weeks of use (he apparently got a lemon). I hope you see my point: There are so many factors involved in determining the lifespan of a certain device that it is impossible to say what will happen to your iPad: Sometimes you get lucky, but there are no guarantees.

With regard to your other 2 questions:


As far as screen life is concerned, 'screen burn' only occurs on the old-style (CRT) monitors. Also, the backlight is provided by LEDs which are supposed to have a pretty long lifespan.
As far as battery life is concerned, there are some pointers at http://www.apple.com/batteries/ipad.html and at http://thehowto.wikidot.com/prolong-battery-lifespan-for-ipad on how to maximize your battery lifespan.


I hope this helps.