 al-Shirazi believes a maximum of 17.5 hour fast from Fajr (and minimum of 6 hours fast which might apply in some northern areas in winter).This is based on an interpretation of what the "urf" (common) is, and understanding the terms "layl" (night) and "nahar" (day) when fasting is referred to in the Quran and Hadith, i.e.what a normal person would understand when hearing these words.