This one is extremely hard to quantify. Ive managed for a handful of organizations and each does it a little differently. It really depends on your industry and reason for having a community. 

If the community is public or private will be the biggest indicator for metrics. If its closed you should have higher % of engaged people vs a public space where people come from Google ect.

If your community is around support the need to people to post isnt always there. People come to read and solve a problem then leave. Most wont engage for these purposes. You will likely see more newer topics with a few less replies heavy focus on solutions. 
If you community is around brand recognition and more than a support portal then engagement would likely be higher. Usually more engaging in terms of replies and less focus on solutions.

I like to monitor solutions and solution views for support communities. Replies, new registrations and views onto event boards for more marketing/community focused communities.

To better answer your direct question, dormant users I am not concerned about in public communities. Many of those accounts turn out to be spam accounts. For private communities, I would focus on churn at the org. Historically, someone who hasnt logged in for a long period of time is susceptible to churn.