When to replace MC Cartridge

Hi all,

What are the reasons to replace a moving coil cartridge and how does one observe them mechanically?

I've seen the estimates for number of hours and how the acoustic properties muddy. In particular I'm looking for what I should see under a very good microscope, and what happens to various frequency ranges as a function of mechanical changes. Should I see burs, asymmetry, flattening of the tip?

I'm using a SUMIKO blue point special with the defacto elliptical stylus.