If you use sand, don't use clumping sand, as it's likely not good for the ferrets' airways. Wood chips is totally wrong for ferrets, either as living material or litter box filling. We've got four boxes where we live, which isn't a large place, and I usually empty two of them twice a day. There's thus always one suitable for everone. When I empty them, I don't empty them totally, I only remove what needs to be removed, usually that closest to the wall, and I move the rest of the filling in that direction.
Material only needs to be added once or twice per week.
In front of some of the boxes are heavy things, so they can't move the boxes and use the area between them and the walls, which apparently is a completely reasonable thing to do. Yes, I've found this to work better than advice like only letting the ferrets be in a limited area to begin with, and then expand it more and more. When Carmen and Rustan joined us, a whole week went by without any of all four using anything but the boxes. Since then, we've had periods of more than two months when noone has missed. Roxane hasn't been staying with us for very long, but she seems at least as well litter trained as the older ones.