The number forty is biblically significant (Jesus was in the wilderness for forty days, the Israelites for forty years). In alchemy the period of forty days is less a precise than an approximate term.
Abraham establishes forty days as the canonical alchemical duration of putrefactio, linking it explicitly to its biblical antecedents while noting its approximate rather than literal character within laboratory and symbolic practice.
, A Dictionary of Alchemical Imagery, 1998thesis