Like others stated they can go months prior to poo-ing. Do not worry about the substrate as this happened to ours when they were on a loose substrate. We did end up putting down paper towels prior to feeding and that worked awesome for us. Loose substrate was a pain in the bootey on feeding day as we were picking stuff out of there mouth with a missed strike. I might suggest prekilling right before giving i to the snake...that way you can control that it will be on the paper towel. Hope that helps.