I've seen a number of sites that have this, and it would be a lot easier than going through a persons edits (which will time out if it was long enough ago) to figure out when a user was last active.
Basically, it would detail when a user last used the credentials for that account (to include session credentials). If it's within a certain time period... say within 5 minutes, then it would list that user as being online.
I'm not sure if API usage should trigger the above though...?
Edit:
- (2017-08-23) Changed title from "User Last Seen At" to "User's Last Edit At"
Updated