So I finally got around to changing the filter media in the 190gal's Marineland C530 today, after 4 months and 20 days. The reason was I'm re-starting the 125gal goldfish tank because it's almost time to bring them in for the winter and get the pond shut down and put away for the season. A half teaspoon of pure ammonium chloride slowly added over the past few days along with some food got the ammonia to about 2ppm, so it was time to add a good dose of BB from the C530's old filter media and get it cycled.
The C530 was still flowing good, but did need changed. Water pressure was pushing up on the bottom section enough to make a bulge in the plastic separator screen. I used a different type of recycled polyfill in that bottom section this time. It seems a bit more dense, so I imagine it won't last that long next time around. The whole point is a filter maintenance schedule should be a moving target with alot of variables. Go ahead and do it every week or 2 or even just once a month if you like, but you're probably wasting time and money if you do. The best approach is to monitor flow and figure out what your particular setup needs.