Updated: Sep 29, 2020
Rabbits usually use one cage corner as the bathroom area. Observe which corner has the buildup under the cage and try to place the nest box in the opposite corner. This helps prevent the doe from fowling the nest box prior to the litter arriving. Be observant that a doe has not wet down a nest box prior to the litter, as this destroys the insulating qualities for the filler material and can cause loss of the litter. Be especially vigilant about this during cold weather, as new born kits need to be kept both dry and warm.
Most rabbit gestations go 30 to 31 days. I like to put the nest box in at 28 days. This gives the doe adequate time to prepare the nest, but also deters her from sitting in the box and mashing down the filler material. If you use a grass or straw nesting material, check the box the next day after placing it in the cage. Many rabbits love to make a meal of either of these fillers (which is very good for the animal at this point) but you may need to replenish the supply. Don't short a doe on nesting material to work with.