You should clean them by rinsing them in saltwater at least weekly perhaps even twice a week if they are really dirty, Then you should replace the filter pads once a month. Only replace on filter pad at a time on your emporer because there is some beneficial bacteria on them. Many people don't even use the filter pads on a continual basis, It really isn't necessary to have them in on all the time, you could put them in for a week once or twice a month.
Well, I guess that was wrong, it really depends on the system, but on a heavily stocked, heavily overfed system, which is the norm, I would go every other week.