In 1997 Jakob Nielsen told us about how users read on the web. (Actually in a typically headline grabbing way, he says they don't and then tells us how they do!)
Nielsen is a big proponent of using bold text - often with several items marked up in this way per paragraph (despite also telling us to use one idea per paragraph).
There is always going to be an argument that it depends on context, so to put the question into a context we can all see, I'm specifically thinking about questions and answers here on stack exchange where there are frequently long bodies of text, but also bearing in mind that visitors are short on time and need to get the 'takeaways' clearly and succinctly.
Clearly, some markup helps with conveying ideas and making the points easily scannable, but on the other hand for those wanting to read an article in full, there can be too much markup that it makes the text harder to read - especially for the (ahem) older readers who find some aspects of readability increasingly more of a problem. Therefore, the effectiveness of markup depends on the reader as much as the writer's style.
There are, of course, a huge number of factors that affect the desire and ability to read or scan text, but is there any modern research (or examples) for the web that show, for most readers, how much text markup is just right?.