This object gives the number of previous 15 minute
intervals for which valid performance monitoring data
has been stored on the interface.
The value of this object will be n (where n is the maximum
number of 15 minute intervals supported at this interface),
unless the measurement was (re-)started within the last
(nx15) minutes, in which case the value will be the
number of previous 15 minute intervals for which the agent
has some data.