Developing an elite swing takes a LONG time! How quickly one develops a high-level swing depends on lots of factors. Age, athletic ability, strength, hand-eye coordination, overcoming poor habits, how hard the player works at it, etc.
To me, a good indication of when you kid "gets it" is when the swing you see off the tee looks like the swing against good pitching. IOW, when facing good pitching, do the proper swing mechanics hold up OR do they revert back to the old hitting pattern? If the swing holds up...then you can back off of hitting lessons. JMHO
Keith