*The achievement seems to have been fixed by the most recent update (12/9/14), so if you had problems before try it again and see if it works.
This achievement is not cumulative, it must be done all at once.
For this to unlock, you must watch a single stream without backing out of it for a whole 8 hours. If you back out unintentionally or on purpose (or it says the broadcast finished when it didn't), you will need to start over. Most streams don't last for 8 hours at a time, so you'll need to find someone who's doing a marathon and let it run for 8 hours.
TwitchPlaysPokemon is a great channel to try and get this on since they are usually streaming for long periods of time.