A new magazine scan reveals that Tales of Xillia will have over 50 hours of gameplay. This scan is from Otona Fami, and states that playing the main game and its scenario, not including completing side quests and such, will clock over 50 hours. If all the other elements are considered, gameplay will clock at least 100 hours.
To compare with the previous games, Tales of Vesperia for the Xbox 360 was given an estimated completion time of 50-60 hours for the main scenario and 100 for completing everything else. Tales of Graces for the Wii was given 40-45 hours for the main and 80 for the full completion, while Graces f for the PS3 was given a higher estimate of 60 hours main scenario time and 100 hours full completion time.
With those estimates, Tales of Xillia seems to be as long as the average Tales game. Or at least, as long as the latest ones, anyway.