A useful starting point is calculating your actual monthly essential expenses — not your current full lifestyle spending, but what you'd genuinely need during the break itself, which may be lower if certain work-related costs (commute, professional wardrobe, eating out during work hours) disappear during the pause. Multiply that by the expected duration of the break, and add a meaningful buffer, since breaks often run longer than initially planned.
It's also worth separating this career-break fund from your regular emergency fund and other financial goals entirely — dipping into retirement savings or a house down payment fund to cover a planned break can quietly derail those other goals, so a dedicated, ring-fenced fund built up in advance specifically for this purpose avoids that trade-off.
Beyond the pure savings number, it's worth factoring in how the break might affect health insurance (if it's currently employer-provided), ongoing EPF or NPS contributions (which will pause along with your salary), and a realistic plan for re-entering the workforce afterward — the financial planning matters, but so does planning for the transition back.