Either wait or skip to the part when Judy Garland(Dorothy) is stuck in the witch's tower and the hourglass is tipped over. Notice when it changes scenes,the hour glass stays the same after two minutes. Also, on the right of the hourglass at the top all through the duration, there is a ripple in the sand. That can't stay there!
I noticed that too, but I don't think they wanted Dorothy to die in the movie, so they made the hourglass seem to go on longer than it should. You know, the whole dramatic "Will She Make It Out Alive" thing.