I tracked the last 15 levels by date, and it's straightforward to see how long each level took me:
Not sure what explains the almost six-month gap to Level 28 - I guess that was the height of my Duolingo fixation?
Also the last few levels were distinctly slower.
Well, now that I've hit the top level, I don't think I will continue playing it as regularly. There are still fun little side quests that you can do, but I never got into the social aspect; it was something to do in the non-reading parts of my commute. Now I'll go back to Big Finish and maybe find some podcasts instead.