Approximately 15% of the time spent watching YouTube is on TVs

"Technology has evolved in the last few years to give viewers a lot of convenient ways to watch internet video on a television screen.
Dedicated digital media players like Roku devices and Apple TVs have been around for years, and it's possible to stream digital video from most video game consoles. More recently, people have been adopting devices like the Google Chromecast, which let users stream video from their mobile phone or computer over a WiFi connection to their TV sets.
YouTube says more people are taking advantage of these options. According to the company, people now watch an average 150 million hours of YouTube per day on TV screens, or about 15 percent of the overall time spent watching YouTube. An Ipsos study commissioned by YouTube found that seven out of 10 of its viewers were watching YouTube on TV screens at least some of the time.
"TV is the fastest growing surface for YouTube viewership," Debbie Weinstein, managing director for YouTube video global solutions, told CNBC.
This viewership isn't replacing mobile viewership, but is rather adding to it, with many using mobile devices as a bridge.
"A lot of the viewership on TV is casting from the mobile device," Weinstein said. "I think it's more about technology and how it interacts in their [users'] lives."
YouTube TV viewers tend to stick around longer than viewers on other platforms, too, the company said. The average mobile YouTube viewing session is 60 minutes, while on TV it's much closer to the typical television watching duration of 4.5 hours. (YouTube declined to give a precise time)."

Source:  CNBC, 30th April 2018