This value indicates how many hours a reel is viewed on average by all users. The bar graph shows the comparison to influencers with a similar number of followers. Some reels are only a few seconds long, while other reels can last up to a minute. This difference is not apparent in the play figures, but is taken into account in the "Watch Time". For campaigns, it can therefore be interesting to see how many hours a sponsored reel is actually played on screens.
Calculation: reel (plays which were watched to the end) * reel (duration)