I was preparing for CAT full-time and gave NMAT in October. Honestly, if you're already in good shape for CAT (especially QA and LRDI), 2–3 weeks of focused NMAT prep is enough. What’s different in NMAT is Speed > Depth (you get less than a minute per question),Adaptive pattern (can't skip questions, can't go back) and Language Skills is tougher than CAT’s VARC, vocab and grammar matter more. 

I was targeting both CAT and NMAT last year. My CAT prep was heavy on concepts, but I realized NMAT is more about speed, pattern familiarity, and accuracy. I spent around 15 days post-CAT prep (1–1.5 hrs/day) focusing just on NMAT Language Skills, Word usage, grammar rules, para jumbles, Quant, Mental math, DI sets under time pressure, LR, Simple but fast  blood relations, input-output, decision making. Also, NMAT allows 3 attempts, so you can improve with each. But if you’re consistent in mocks and score 220+ in practice, 10-14 days of focused prep is enough.

I gave 6 NMAT mocks, practiced sectional tests, and focused on speed-based sets. If your CAT prep is solid, just add vocab flashcards and mock-based tuning for NMAT and you're good.