There's no specific time, I always try it to be the less possible but sometimes it may take a bit longer than usual. It also depends on the movie and how the picture came. Having the pictures tagged and in the correct season/episode when it's a TV-show helps. Sometimes even with that some may take longer, as yours, because they usually go to movies that have many pictures, so I have to first try to see if there is any duplicate of them, and after that, comparing both pictures to see which one is actually better (not just approving by trusting the numbers).
Sorry about delay this week, I'm a bit more busy outside the website and it certainly affected the good rythm of approval I've had last month when everything was smoother.