When you can't see into the refugium!
I guess it also depends on what you have in the refugium, and what you're trying to maintain. Eventually chaeto growth can get so large that it can block out more chaeto growth in the center/bottom if it's not spun. And to keep Caulerpa from going sexual you have to prune it down, sometimes quite heavily. I think miracle mud has an actual lifetime that you need to replace it by, and that would probably work well for any sand in the fuge as well for export of detritus. I doubt a deep clean would ever be helpful unless starting over from scratch; smaller cleans would probably be better.