NDA examination is an examination conducted by UPSC to induct cadets in Army, Navy and Air force. The syllabus for the examination is Mathematics upto 10th standard, English upto 10th and Questions on general knowledge and current affairs.  Only students who are appearing in their 12th and who have passed 12th can apply for the exam and the age criteria is also from 16 1/2 to 19. Now, answering your question. One should start preparing for NDA exam depends on the calibre of the student . for example students getting more than 85% marks in maths and english in 10th standard board exam just need one month of general knowledge study and can easily crack the exam. People who are quiet weak in maths should start 3 months before the exam and proper disciplined practice will be more than enough. But, if you are really skeptical about your knowledge, starting preparation six months before exam will be the safest for you. One thing I would like to add that, NDA exam is not as important as SSB which is conducted after exam. SSB is your real test of your personality if you are really aspiring to join the forces. Best of luck  

Posted By: Nilesh