From French YouTuber Julien Chièze, who managed to bag an interview with Ubisoft lead producer Fabian Salomon. They discuss Assassin’s Creed Mirage’s average run time at around 17 minutes, with PCGamesN giving the following translation from French:

“Given that we do a lot of playtesting internally at Ubisoft, it’s part of our process, we really want to get as close as possible to the players, so we’ll say that the latest playtimes we’ve received average at around 20-23 hours,” Salomon says. “That can go up to 25-30 hours for the completionists, and we’ll say that those who will be rushing the game will be around 20 hours.”

Definitely relieved to hear it won't be another 100-hour epic, but ~25 hours is a surprising reduction!