Excess black residue

Well, in this case, its a major undertaking to pull the fan. This is an insert and it has to come out for me to access it. Its a job just to get to it and then putting it back in, in the exact right spot so the surround fits right. Its a lot of work so I wont be doing it unless I need to. It was only a little dirty after 3 years so I may clean it every 2 years but definately not every year.