When will I be able to fill the form up for 2018 to the following exam- MAT, CAT, CMAT, XAT, GMAT and SNAP?

It is possible but not advisable, as you may not score as desired. It is important to cover the SNAP syllabus too. You can solve questions in mock test correctly only if you understand the topics well. 

• For every correct option 1 mark is awarded
• Forr every incorrect option 0.25 marks are deducted
• No deduction for unanswered questions

SNAP exam is a computer based test mode. Candidates have to visit the allotted test centre and take the test on a computer system. 

Yes, six months are enough to prepare for SNAP exam. Candidates should start preparation by June to be well prepared by December, the exam month.