Learning how to drive and becoming a good driver are 2 different things.... Many people can learn to drive in few hours and start driving on normal roads... But being a good driver takes time and experience, i.e. learning and understanding gradually what action to take at what point... A new driver would simply stand on brakes in case of emergency while a good\experienced driver wont be in that position in first place, even if he is, then he could handle it professionally.
Per me, there's no fixed time period in which you can be a good driver.. can vary for everyone differently.
No one taught me to drive, i just watched my father and learned... On my first time at 16yrs he just sat beside me without saying a word while i seamlessly drove without an error... Then got my hand back on car at 19 when i got my license.... I can say I am a good driver for the type of traffic in Pakistan.. but wont say same for traffic anywhere else in world