It's also very important to take into consideration how much your loads are stretching your cases. In addition to the number of firings I also track and consider how may times I've trimmed my brass.
Somewhere along my 50 years of reloading, I picked up a rule of thumb to discard casings after 4 trimmings. I don't know how valid this is or how it was determined...but I adhere to it in addition to other factors that I consider during my case examination process.
I also routinely discard cases with 10 firings.