Xbox One owners have played over 500 million hours of backward compatible games

[SvennoJ]

Is that lifetime running total since BC came out?

It still doesn't negate the snapshot of ars technica over a period of 5 months.

https://www.vg247.com/2017/01/05/microsoft-says-xbox-live-engagement-reached-an-all-time-high-of-3-9-billion-hours-in-2016/
During November and December 2016 Xbox Live engagement reached an “all-time high” with 3.9 billion hours spent using the service. Xbox One owners spent 265 million hours playing Xbox 360 games in 2016 via Xbox One Backward Compatibility

Now I guess during the last 2 months of the year the system is played more than the rest of the year, so lets take a very conservative estimate of average 1 billion xbl hours per month, so total for 2016 would be something like 14 billion hours. 265 million out of 14 billion = 1.9% Extrapolate the 1.95 billion hours per month and it's 1.1%

So time spent on XBox BC is between 1.1% and 1.9%

Yet the 'good' thing is, it's growing, not diminishing. Another 235 million hours for the first half of 2017.
Good is relative ofcourse, more time spend on old games, less revenue made. It's all borderline insignificant anyway, 2%, 3%, tiny piece of the pie.