ICAO Annex 6 states:
"1) for a reciprocating engine aeroplane, the amount of fuel required to fly for 45 minutes, under speed and altitude conditions specified by the State of the Operator; or
2) for a turbine-engined aeroplane, the amount of fuel required to fly for 30 minutes at holding speed at 450 m (1 500 ft) above aerodrome elevation in standard conditions"
The 30 minutes for a turbine airplane have always been considered at holding speed 1500ft above the airfield, while the 45 minutes for reciprocating-engined airplanes have been considered for a long time at cruise level and cruise speed (although this has been changed last year in EASA land).