Just to kick it off,
i shot over my pup recently, she was 6 months, i know this is too young for some people, but i felt she was ready, we hunted up 2 or 3 rabbits, i shot 1, she made the retrieve and then we went home, all in the name of training.
The whole session was only 1 hour, which i think was enough for her age.
That was 4 weeks ago now and we haven't repeated the experience again yet, even though she did very well.
We have just carried on with further training and i will shoot over her, for another hour maybe next week, taking it slow, building up her experience and her confiden