Google’s John Mueller responded to a thread on Reddit saying that having too many heading sections, like as deep as H6, can be a waste of time for SEO. He said “Having this many sections is almost never going to make sense, and changing anything in the last level is never going to change anything with regards to SEO. This is a waste of time when it comes to SEO.”
He said it is mostly common sense. Someone else said “In this case, changing between a <h6> or a <span> or a <h2> will not lead to any obvious increase in rankings.” John responded “Exactly!”
He then added ” It’s obvious when you just look at the numbers… Assuming you have 3 sub-sections per section, that would be: 1xH1 (insert argument about number of H1s here) * 3xH2 * 3xH3 * 3xH4 * 3xH5 * 3xH6 = 243 headings / sections on this page.”
Forum discussion at Reddit.