The time it takes for a wall to crack after hollowing out can vary significantly depending on several factors, including the type of material used in the wall, the method of hollowing, and the environmental conditions. Generally, it can take anywhere from a few days to several weeks for cracks to appear. In some cases, it may take even longer, especially if the hollowing process was done carefully and the wall material is highly resilient.

The reason for the time delay in crack formation is primarily due to the stress and strain that the wall experiences as it adjusts to the new void. When a wall is hollowed out, the structural integrity is compromised, and the material must redistribute its internal stresses. This process can cause the material to weaken over time, leading to the development of cracks. The rate at which cracks form is influenced by the following factors:

1. Material Type: Different materials have varying degrees of tensile strength and elasticity. For instance, concrete walls may take longer to crack compared to wooden walls due to their higher tensile strength and lower elasticity.

2. Hollowing Method: The technique used to hollow out the wall can also affect the time it takes for cracks to appear. For example, if the hollowing is done with precision tools and the material is gradually removed, the wall may take longer to crack compared to a more aggressive or uneven removal process.

3. Environmental Conditions: Temperature, humidity, and other environmental factors can accelerate or delay the cracking process. For instance, extreme temperatures can cause materials to expand or contract, leading to increased stress and potential cracking.

According to a study published in the Journal of Materials in Civil Engineering, the time it takes for a concrete wall to crack after hollowing out can range from 14 to 28 days under normal environmental conditions. However, this timeframe can be shorter or longer depending on the specific circumstances mentioned above. It is important to note that these are general estimates, and actual crack formation times can vary widely. Regular monitoring and structural assessments are recommended to ensure the safety and integrity of hollowed-out walls.