Great question.
This means that if somebody (like me) replied to your post in ABC topic, and the minutes that elapsed between your post and my reply were 180 (and nobody else had replied before me) then the average is 180.
However, if there were no other posts in the community that month (an unlikely scenario, but indulge me on this one) except for you starting the same exact Topic in a different board (hoping that more people would see), and I saw that Topic 8 days later and replied with a link to the original ABC topic that I had replied to 180 minutes after you had first started it, LSI is ignoring my 8 day late response.
Hopefully this makes sense. It may seem kind of arbitrary (and it somewhat is) that we selected 7 days, but most Topics run their lifecycle in 2-4 days, and so 7 days is generous. Of course there are many times that we see somebody coming back to (or first discovering) a Topic that is old and posting in it long after the most recent post. In forum etiquette this is called 'necromancy' which is a more traditional way of talking about 'raising the dead'. Some communities have rules against that.
Minutes to First Reply is a great metric to look at. In fact, I would consider it in my top 10 metrics (but not the top 8). It is one of the very few metrics where a 'lower number' is better, and it is also a 'noisy metric' in that if you dont put limits on it (like the 7 day limit that we have in LSI), it becomes all but useless in helping to inform you to the true nature of your community.
Hope this helps!
