YouTube streams more than six billion hours of video a month. Soon it will let people watch some of those videos even if they’re not connected to the Web. Next month, Google’s video site will let viewers save clips on their phones and other mobile devices for up to 48 hours, so they can watch them when they’re not online. The videos will still be free, and Google will run ads on the clips, which will be available via its mobile apps. That’s a big change for the eight-year-old company, which has previously required a live Web connection to watch its videos (though there are plenty of workarounds). YouTube announced the move in a blog post published late last night, and started telling its partners about its plans via email, as well; YouTube says it will allow uploaders to opt out of the offline option.