Mentioned only here and in Quintilian 12.10.21, where he seems to be a contemporary of Lysias and Andocides, i.e. as much in the C5 as the C4 BCE. Any "pupil of Isocrates" would need, on the face of it, to be a generation later. But in fact the name is unique in Attic prosopography - see LGPN ii s.v. - and there has to be a serious possibility that it is not a proper name but a nickname (though for whom?); LSJ s.v. ko/kkos offers various possibilities (web address 1 below), i.e. Seedy, Weedy.
[1] See iota 652.