What is the minimum length of the wall after which expansion joint is to be provided?

Expansion Joint will be provided as per the material used for construction of wall(RCC Wall,Masonry wall).

However Length is calculated using coefficient of thermal expansion of the material , the temperature variations in that location and permissible strain in the material

for every45m an expansion joint should be provided for RCC structure as per Indian standard codes

Minimum length is40 meters

it depends on the location weather. how much the temperature varies between day and night, summer and winter.

But usually its around every15m. You can also provide control joint every6m.