How long does it take to learn to kick over from a back bend?

This depends greatly upon how flexible you are and how often/hard you work at it. You could easily learn how to do so with proper exercise and spotting in a week or two but if you are a good ways from your goal it could take a month or two. When you practice, start facing a wall then do your back bend,slowly climb your feet up the wall and kick over. That will help you learn what it feels like and will help build the flexibility in your back.