I never do a full bedding change, you lose too many eggs and hatchlings that way in my opinion. I sift it out about once a month or so to get the worst of the droppings and tiny leftovers out, and then ad a cup or two more of oats or wheat bran, whichever I happen to have on hand. About once a month I throw in a small handful of fish flakes or dry dog food too. I use the siftings as a starter culture for getting the new bed going- I rotate three bins this way. The siftings almost always have eggs and new hatchlings in with them.