It used to say somewhere under hedgehog homes, but I can’t find it, so maybe it isn’t there any more, but I seem to remember that it said Spring and Autumn. But it’s best not to disturb them if someone is actually in the box. It’s a bit tricky if they are still in regular use. In the wild they would probably move to another nest if they found the one they were in became too infested, etc. So you would think they would move out at some stage. They shouldn’t get as bad as the ones in care, because those would be pooing all over the place and probably spilling food and water everywhere! Have you got any spare houses, they might move into, so could clean the old ones out?
If you find they are out during the day, I would be inclined to clean them out now. I know last year I kept getting a lot of ticks in one of the boxes and they were only using it for naps between snacks.