Qing Piaoyun was keeping a close watch on Lin Xuan's situation. He wanted to see just how long Lin Xuan could last. Naturally, the longer he stayed, the better the effect would be. The purpose of the Bone Forging Pool was to re-forge the bones within one's body. To put it bluntly, the Bone Forging Liquid would flood the entire body and merge with the bones.
This process was inevitably painful. The moment the Bone Forging Liquid entered the body, it would shatter the surface of the bones and then seep into the marrow, strengthening them until they were comparable in hardness to Dragon Bone.
The reason Qing Piaoyun had said not to stay for less than two days was because two days wasn't enough to replace all the bones in the body, but it was enough to replace a small portion—or rather, truly replace a small portion. Anything less than a day would only result in a surface-level replacement, not even swapping out half a bone, making it a complete waste.
