I flushed mine at 97,000 miles and it was pretty darn filthy. It made my reservoir look like a windshield washer reservoir it was so clean. Also made a noticeable difference in engine temps.
I only drive my car in the summer, at the end of the season I dump 4L and add 4L fresh. I figure that way the quality of my coolant remains pretty consistent. I think this help with the fact that my car gets stored yearly as well. I also do it in my wife's cx5, yearly.