Usually the time in microseconds that other stations should treat the medium as busy. In some control frames it carries an association identifier instead.
This is what sets the network allocation vector on every station that hears the frame, whether or not they can decode the rest of it. Because it is little-endian, the octets 2C 00 mean 44 microseconds — reading them the other way round gives 11264, which is the classic byte-order mistake.
QoS Data MPDU > Duration/ID