When I used one 24/7/365 for my 60g cube, it was a fluval 405 with coarse and fine sponges, then carbon, then LR rubble, then more carbon. It was not a nitrate factory despite what some may say. I cleaned it when I did water changes, every 4 - 6 weeks, or some 265 times over it's 22 year lifespan.
It has been retired (well semi-retired) now. It's great if I need to do a big cleanup - I stuff it full of polyester pillow stuffing and let it run for a day or 2 to clear up whatever I've stirred up.