Navkarsi is timed as a fixed window after sunrise.

• How it is calculated: Navkarsi begins about 48 minutes after local sunrise. So you wake up, complete morning purity rites, and then start eating/drinking after that 48-minute gap. The exact clock time depends on your city’s sunrise on that date. Some sanghs may use a slightly different gap, but 48 minutes after sunrise is the common rule. ( jainknowledge.com)

• Quick example: If sunrise in your city is at 6:20 AM, Navkarsi would be around 7:08 AM (roughly 48 minutes after 6:20). If sunrise changes day to day, simply add 48 minutes to that day’s sunrise time.

• Traditions (Digambar/Shwetambar): There is no separate traditional rule for Navkarsi timing between Digambar and Shwetambar; both follow the same 48-minute-after-sunrise guideline in practice, with minor local variation by sangh. If your sangh specifies a different offset, follow that local practice.

You can read more here and here .

Note: For precise timing in your area, check with your temple or local panchang.