I am writing this answer from my personal experience.
From which year during MBBS should you start preparing for NEET PG ? The answer is NONE.
HERE IT IS WHY :
Till you come to the final year, whatever you have studied you will very easily forget. Consider that you have really good memory and you do not forget what you have read in previous years, my dear all of whatever you have read gets updated/changed.
Anatomy : An artery stays as an artery, A muscle remains as muscle but the course of nerve changes in skull as per the current research going on. 3 layers in embryology ectoderm mesoderm endoderm. Now there is 4th layer : Neural Crest Cells.
MCQ are asked from the Neural Crest Cells.
In the same way, in Pharma new drugs are being approved and experimented. MCQ comes as Recently approved drug for XYZ is ???
The statistics in Community Medicine changes every 6 months.
Medicine and Surgery you can never complete
There are a lot of questions from Anesthesia (beyond pharma drugs) Psychiatry, Dermatology (most ignored ), Radiology (never visited).
Coming to Internship : the most beautiful time in the whole of MEDICAL CAREER : No Liability, Full Freedom, No Restrictions : Enjoy the Internship and Use it make the best memories with the friends.
It is not compulsory for everyone to do PG after MBBS.
In Delhi a Junior Resident (MBBS) gets paid ₹ 97910 per month (without tax deduction). Similarly just by being an MBBS in other private hospitals too you can earn around ₹ 50000 easily.
In southern part of India MBBS junior doctors are paid bit less in private hospitals but they can make good salary in govt. hospitals/PHC.
After Internship : Take a break from studies : Work somewhere and experience what is working in real life : Do the savings : Pay for your coaching : Read and give effort for PG preparation for 1 year and you can crack NEET PG easily.
I see many students coming in the coaching classes in the name of foundation course. I feel pity for them. They are wasting precious years of good time which should be spent on making good memories with friends.
Years after from now when you sit at home, a notification from Facebook about a memory of few years back having a pic of yours with your best friends of a trip will really bring a big smile on your face rather than the marks you scored in NEET PG.