Generally, 7-8 months is a decent enough time to commence CAT preparation. So, if the CAT exam is held in November as it usually is, CAT preparation should start in March itself.
This however is a general rule. Some students prepare for 12 months or more while some start as late as five months before the exam. 7-8 months hits a sweet spot as it leaves enough time for revision and review.
A longer preparation isnâ€™t necessarily as advisable as candidates often run out of steam in the last five months of preparation. Thus, Iâ€™d suggest you start your CAT 2018 prep in March.
Best of Luck!