I think the topic is too broad and without a serious research it is really difficult to give any answers. IMO, all the contributors were put in a situation where they had to do some wild guesses and that's why the answers are so different.

What is sure is that it does not depend on constant factors, such as keyword or categories used by the businesses, or relatively constant, like number of competitors for a particular keyword. The reason why I think so, is because an ever-changing SERP (from Blended to Pure) is evident for the past few months.

I am planning to do a massive research on the problem, but the major factor that is making its exactness difficult, is that Google is changing their algorithm very often + making major modifications such as pictures not appearing in the organic search results (a recent one).