It takes at least 50-100g of carbs per day to prevent ketosis.116.
Fat supplies 60% of the body's ongoing energy needs during rest. Even on a total fast a person cannot lose more than half a pound of pure fat per day.158
Now down to what you guys are talking about.
Fasting | Vhi Healthcare
Appears to be the third day for men and second day for women. This will vary depending on your activity levels and how full your glycogen stores were. The third and second days are assuming full glycogen stores (375-500g) of carbs. The brain requires 4-600 calories of glucose per day.
It definitely won't take weeks. A few days is probably the most accurate answer.
When ketones rise in the blood the ph drops and becomes more acidic.
If you can't tell I'm not a fan!
1. Whitney, Eleanor and Sharon Rolfes. Understanding Nutrition. Belmont: Thomson/Wadsworth, 2005.