The deposition of secondary dentin is a slow and continuous process that occurs after the completion of root formation. It typically requires 8-12 weeks for noticeable formation because odontoblasts gradually lay down dentin in response to normal physiological stimuli or mild irritation. This time frame allows for the gradual thickening of dentin, which protects the pulp chamber without compromising tooth integrity. Rapid deposition, as seen in tertiary dentin, occurs only in response to severe injury, which is why shorter durations (1-4 weeks) do not apply here.

Reference: Oral Histology, B.K. B. Berkovitz, Edition 5.