NIOS is the university run by the Indian Government by Minister of human resource development.If you have passed your 12th from other university/Board,not from NIOS,You are not allowed to give the exam of mathematics directly.If you want to give mathematics exam, you need to repeat class 11th again,after that you will be eligible to give mathematics exam from NIOS.But you can try in NIOS  through demand examination policy.For this you have to register the form to apply for on-demand examination of mathematics through their website(www.nios.ac.in) or contacting NIOS offices.NiOS syllabus is large but conceptuality as compared to other boards,But their copy checking is very strict.It is not time consuming because you did not have to prepare for board examination.You just need to study 11th and optional module completely and just the concepts . Minimum 3 months are required for preparation of mathematics.