Plastering work should be levelled, plumbed up and the joints should be at right angle at the corners or edges.
It must cover the non-verticality and non-horizontality of walls and columns which are resulting from poor workmanship and non-standard bricks.
Generally, the plaster thickness should not exceed 15 mm. Else the mortar bulk might tend to fall due to its self-weight and thus the cracks can be initiated in later stages.
The mortar mix proportion of cement: sand = 1:5 is generally used for the wall plastering.