I had the same problem yesterday, when the game did show around 1060 minutes (so, more than 17 hours) of turnaround time for my only plane currently in service. This happened at about 16:00 local time and the plane was scheduled to depart the airport again at 20:45 - which it did without any difficulties, as far as I can tell.
Therefore, I assume it's only a visual bug (and it's actually showing, when the plane will be available again, after the whole route with every single leg has been finished).
Therefore, I assume it's only a visual bug (and it's actually showing, when the plane will be available again, after the whole route with every single leg has been finished).