Speaking on Google's Q2 2015 earnings call (broadcast on YouTube, as you'd expect), new CFO Ruth Porat dropped some new numbers about how YouTube is growing. Total watch time on YouTube is up 60 percent from this time last year, the strongest growth rate Google has seen in two years. Most interestingly, mobile watch time has also doubled in the last year — CBO Omid Kordestani added that the average viewing session is over 40 minutes long on mobile.
With new YouTube initiatives like YouTube Gaming and YouTube Kids expanding the platform to even more users, you can expect to see numbers continue to grow. It likely doesn't hurt that phone screens are getting larger either, making it easier to watch longer-form content on the service.