Hi Lily of the Valley
Normally twice a year is recommended in Spring and Autumn.
https://www.hedgehogstreet.org/cleaning-out-boxes/
But if you know the house is empty you could clean it other times, especially if you know or suspect it has ticks in it. But the house must be completely vacated – i.e. not the hog just gone out for the night.
Bearing in mind it isn’t quite autumn (astronomically) I would wait and see whether that hog moves out and if it does, give the box a clean out then. If it stays, you should not disturb it, so may have to wait until Spring.
The best thing to clean the houses with is boiling water, so that it kills any parasite eggs which may be hiding in the cracks.
