I haven't left mine on, but, from my LED experience, LEDs generally don't burn out so much as, over time, their output diminishes. I think that the "10,000 hour" life that some LEDs are quoted as having is the time that they drop to 50% output. A 365 day year has 8,760 hours in it.
That also depends on how hard the LEDs are driven. So, if you left your backlight on 24/7, you might notice dimming somewhere around 14 months from now, or longer. YMMV.
I was just thinking that it would be good to know how much current they draw and realized that I could measure the current of a console with the back light off and then on. And then I remembered the Current Draw and Battery Life table in the catalog...
According to the table on p 16 of the 2011 catalog, the back lights draw 160 ma. Now to examine a console and see how many LEDs there are.