Had mine almost 4 years, now. My wife got her red 8100 at the same time as my black one, but she moved to the iPhone last year.
My trackball has lasted me all these years, although I have had to remove it, disassemble it, clean it, reassemble it at least twice a year since I keep it in my pocket except when I'm at home or when at my desk at the office. I finally had to switch out my trackball for my wife's trackball (since she doesn't use it anymore). Mine is worn down smooth to almost a polish. It just can't seem to grip the 4 toothed rods that turn the magnetic wheels in the trackball assembly anymore. Her trackball still has that fine frosty texture to it (although neither of our trackballs are opaqe white anymore - now they are a nasty brown color from age, but at least the color LEDs shine through when they come on).
I also had to use her back battery cover because mine lost it's little rib that holds it closed. So now, I have the black Blackberry with the red back cover.
I'm just waiting for AT&T to finally get the 9100 Pearl 3G.