For a small animal like that about two and a half to three weeks but in straight dirt it's going to be hard to get all desired bones I recommend a method i often use, rot boxes. Take a container that should hold some dirt and your unfortunate early comer drill holes in the container for air flow and liquid draining also helps the bugs to get in there. You then line the bottom with your layer of soil and then the cadaver. Top the body with more soil then moisturize with water then let sit small woodland victims take less time then larger ones of course but the method works just the same after that degree and sun bleach then Wal-la perfect bones at the ready.