We do a breeding ratio of up to 1.4 (usually fewer, as we take out the obviously pregnant ones and put them in birthing bins, where they stay until their brood is weaned and can reach the food and water) and change the litter in all the bins every 3 days. When they were indoors, it would start to get pretty funky by day 3, because the racks were in a closet, and it compounded a bit over time. Now that they're in the shed, we just flick on the fan, and even on a bad day, the smell is gone in 3 minutes.