Duty limits are defined by a table in OMA and approved by the regulator. Actual figures depend on start time and number of sectors. Night or unacclimatised is typically 11 hours while a mid morning start would be 13 hours. Extension tables are also published which for an extra hour but they come with conditions such as extra rest, only 2 between extended recovery rest periods etc to dissuade their use. So up to 14 hours planned is legal. Plus discretion.