The limits for impurities are set as per the maximum daily dose administered in one day i.e. in a 24 hour cycle.
To respond to your question related to the setting of impurities limits w. r. t the synthetic drug substances, in general you follow the ICH Q3 series of guidelines. Irrespective of the market the limits are set in line to these guidelines. The calculation parts are given at the end of each guideline to follow.
These guidelines tell about how to set the limits (reporting threshold limit, limits for setting known and unknown impurities) for drug substance Q3A, drug product Q3B, Residual solvents Q3C and Elemental impurities Q3D.
Hope these clarifies you doubt, go through these guidelines and you will definitely understand.