Proposed XF suggestion:
Additionally, it could display how long it would take to read any new content posted since the last time you visited that thread, see the 'AndyB's Appreciation thread' example in the image above.
I'm not sure how other sites currently calculate it. If it's simply based on word count, it could be pretty simple to implement a basic version of this.
Of course, people read at different speeds. So for it to be accurate, ideally I'm thinking it may have to track users read time over various threads to figure out an average read speed for each user. Custom read times could then be displayed to each user.
Even the slightly less accurate and lower overhead version based on word count alone as a guide would be rather useful.
Perhaps it could be hidden for small thread, under a certain post count and only display for longer threads, where the read time is longer. This could be time to read threshold settin or a post count setting in the ACP.