If it is used in connection with a period of time about refers to usually 15 days, although each case should be considered on its own merit.
Example Usage: Below examples are collected from well-known standard format time charter party contracts under field description of vessel's "Speed Capability in Knots and Fuel Consumption".
NYPE 1946 lines 9 –10
“... and capable of steaming, fully laden, under good weather conditions about ... knots on a consumption of about ... tons of … best grade fuel oil ...”
NYPE 1993 lines 18-20
... Speed about ... knots, fully laden, in good weather conditions up to and including maximum Force... on the Beaufort wind scale, on a consumption of about ... tons of ...
PART I Box 12
Speed capability in knots (abt.) on a consumption in tons (abt.) of m …
PART II lines 10-13
... and fully loaded capable of steaming about the number of knots indicated in Box 12 in good weather and smooth water on a consumption of about the number of tons fuel oil stated in Box 12 ...