Exposure of silver Chloride to sunlight for a long
duration turns grey due to

Exposure of silver chloride to sunlight for a long duration turns grey due to photolytic decomposition i.e decomposition in the presence of sunlight.

Explanation:

When silver chloride, AgCl is exposed to sunlight for a long time, it will undergo decomposition as the sunlight provides sufficient energy needed to decomposed the salt, AgCl to metallic silver and chlorine gas. This can be seen in the equation below:

2AgCl —> 2Ag + Cl2

6.4 cm (max) - 25 cm (min)