The length of fasting depends on the type and purpose of fasting, and it varies widely from short daily fasts to prolonged therapeutic fasts:

• For beginners or general metabolic health, fasting for 12 to 16 hours daily (such as the 16:8 intermittent fasting method) is common and safe. This involves fasting overnight and skipping some meals within a day, such as from 7 p.m. to 7 a.m. or 9 a.m. to 5 p.m.

• A 24-hour fast, done once or twice a week, is another approach where no food is consumed for a full day but water and non-caloric drinks are allowed. This can help with weight loss and metabolic improvements but may cause hunger and irritability.

• Longer therapeutic fasts, from 5 to 30 days, are practiced in fasting clinics under medical supervision and are aimed at more significant health resetting effects. These require careful preparation and refeeding after the fast.

• Very long fasts (up to around 40 days) are possible but should only be done under strict medical supervision to avoid health risks.

In summary, most people start with intermittent fasting of 12 to 16 hours daily or occasional 24-hour fasts. Longer fasts are for specific therapeutic purposes and are done less frequently under professional guidance.