It's hard to make sweeping generalities about filler longevity vs. thickness. There are just too many variables. However, it has been my experience that if the panel flexes at all, thick filler will eventually fail. A prime example are the 'C' pillars. Eventually the filler will crack and fail. The body just flexes too much at this point. As to the quarters, what is pretty thick? I've seen 1/2" of filler on quarters that survived. You'll just have to see.