Short answer: For a shuddh (proper) Navkarshi, Chauvihar the previous evening is considered the ideal and traditional requirement, but in practice Navkarshi can be done without Chauvihar if there is some genuine difficulty.

In simple points:

1. Ideal / full‑maryādā way

- Evening: Take Chauvihar – no food and no water after sunset (many even stop 48 minutes before sunset). - Next day: Do not eat or drink anything till 48 minutes after sunrise, then do Navkarshi (after Navkar Mantra and Pachchakhān). - This is how Shwetambar and Digambar lay‑followers traditionally define a “proper” Navkarshi.

1. If Chauvihar is not possible

- Because of health, age, medicines, or unavoidable situations, some people cannot keep full Chauvihar. - In that case, you may still observe Navkarshi (no food/drink from early morning till 48 minutes after sunrise). - This gives some punya and discipline, though it is considered less pure than Navkarshi done after full Chauvihar.

1. Minimum gap if Chauvihar not done

- Traditional guidance: keep at least about 9 hours gap between last intake (food/water) at night and Navkarshi time in the morning. - Still, the ideal remains: do Chauvihar whenever you can.

1. Practical understanding

- Not strictly “compulsory” in the sense that if you miss Chauvihar, you are forbidden to do Navkarshi. - Spiritually best and recommended: Always try to do Navkarshi with Chauvihar of the previous evening.

If you want to read more detailed Q&A on this same topic, you can see it here and also a related clarification here