You'd need only 3 or 4 times for a very useful sample with basically the same applicability as 20. If it's a low cost change, sure, you may not even question i

You'd need only 3 or 4 times for a very useful sample with basically the same applicability as 20. If it's a low cost change, sure, you may not even question if it's doing anything. But if it's a major change in lifestyle, I would absolutely try to check if it's working. Even more because if it's only incidentally working, it could stop working at any time and leave you knowing nothing useful right at the point where you are both sick again and paying the cost of that lifestyle change. Do that again and again and you are sure to destroy your life.