Sales do play into it, but I don't do speadsheets or deep analysis of a product beyond new release weekend. It's just a general "haven't seen a sale for this in a while" if anything.
It really all does depend on the designer! Some do spreadsheets, some go by sales. Some are sentimental and cry and say bye.. farewell.. why do you have to go?!
I am the same as Grace. If I feel it no longer represents my style or skill well, I retire it. In the past, I have also retired ones that I was just sick of looking at. LOL