To be fair, I’m the one who hasn’t ever estimated reading time by post count.
So I suppose there are other people like me.
In my mind, there is no big difference between 20 and 30 messages.
But there is some between 7 and 15 minutes.
We can feel time better than the number of messages.
In the first case, I have to calculate / estimate it in my mind.
In the second one, I just get it explicitly.
What is more, it would be interesting to hide the number of posts entirely and show reading time instead, which could make more sense in some cases. I.e. when it comes to 50+ messages, chances are the reader doesn’t care anymore who many more - 60, 70, 120? Does it matter? But when it says 30 minutes or 50 minutes - that could make some sense for the reader.
I caught myself reading more at medium.com just because they inform explicitly that a particular article is only 5 minutes reading or so. So even though the topic may sound not so interesting, why not to give it a try. Then in the middle of the text you find out it was worth starting to read. And you’re more engaged after all. I caught myself doing so, and that’s why proposed this as a feature / plugin for Discourse.
After all, it seems that some topics can have little messages but with really long reflections and write-lover authors engaged, while others can have plenty messages but one paragraph per each.
Conclusion: I’d use it with pleasure if there was such a plugin.