When I watch anything, I typically exit when the credits start to roll. Infuse does not mark the item as watched. Infuse should mark items as watched when, for example, there is a minute left of the item. For reference, Plex behaves this way. It can be rolling estimate, too: 30-minute items could have a two minute threshold, while two-hour items could have a five minute threshold. There could also be a verification if the next item has started playing and the last item had a few minutes left–just mark it watched.
I just realized that this request is essentially a duplicate of point #2 in Watched Status TV show folders & time left.... This thread can be closed or deleted.