I haven't been through as many readers as many people here. I went through the Nook Color, Nook Simple Touch, Nook Simple Touch with Glowlight, Kindle Paperwhite 1, and now the Kindle Paperwhite 2 within a fairly short time. 5 readers in less than 3 years? I guess that's not completely terrible. I gave the NST as a gift, sold the NSTWGL, and am giving the PW1 to my sister for Christmas. So I don't have extra devices everywhere.
I still own the NC and PW2, but only the PW2 is used for reading. The NC sits in a drawer most of the time, but I use it for Netflix in bed sometimes.
I'd like to say I'm not addicted. But honestly, if next September rolls around and Amazon announces a new e-ink device, I'm 95% going to be pre-ordering again. Reading is such a big pastime for me, I justify the expense to myself
For my other devices, I usually wait until I feel like I need a new one. My iPhone 5S replaced a barely-functioning iPhone 3GS. My new Macbook Pro Retina replaced a Dell Inspiron Mini 10 netbook for class use. Basically, when they start frustrating the sanity out of me, I replace!