To determine when to replenish, regulate the thickness and visual appearance on the wood shavings. If they've noticeably decomposed or if weeds are beginning to break by, it’s time for you to increase a fresh layer. On a yearly basis we set aside a sack or two of sawdust https://energyholzgmbh.com/guide-to-pine-and-mixed-wood-shavings-for-animal-bedding/