It will be a judgement call on your part, based on your feeding habits and equipment.
If you run floss or a sock in front of the balls all the time, then you could maybe go a month or more.
If you don't run floss or a sock 24/7, you should wash them every week or two, whenever you do your water change.
In time you may find that they only cause work, because you say hey the tank looks best when they are cleanest, then why not remove them all together?
They really aren't that much different then filter floss in the sense they will also mechanically filter detritus almost immediately, if the detritus is not removed in a reasonable time then it will obviously contribute to nuisance algae etc.
In an ideal world, you would rinse them every day or two, but who wants to do that.