Hello,

For appearing in Class 12th Public Exams of NIOS you can apply eight month before the exam. So, the duration you will have for preparing class 12th is around  eight months.

NIOS Public EXams are held twice a years - that is once in April/May and other in October/November.

Last date of Application for October Session is usually 31st January without late fee. For April Session, Last date of Application is 31st July.

So, it gives you around eight months duration

Click at this link to know more- https://school.careers360.com/exams/nios-class-12

Hope this was helpful.