Since the latest update, Plex has started buffering at around 42 minutes. The last time I saw it, it was at about 42:29, but it’s been fairly consistent across different episodes, servers, and platforms. Additionally, I have verified with friends and it is occurring for them as well, on separate shows, and servers.
The only consistency we’ve found is the version of Plex: 1.11.1.4760.
Edit: Having watched shows of varying durations, it appears to be around the 95% mark, not simply time based.