Best Viewer NA Achievement Guide
This is a cumulative achievement after you have watched 100 hours this achievement will unlock. You can check your progress at anytime by viewing the achievement each percent = an hour
example: 54% = 54 hours
Time will count in snap mode as well, I personally found it easier for the much more time consuming games like Forza 5. You can be proactive as well, and launch twitch and snap another app that has a cumulative achievement as well.
Advice is to find someone doing a marathon video of some kind, go into their video, and leave it running while you go to bed/work to rack up a bunch of time.