Training sessions – how often?

We have a five month old hound/potty mix. She’s a smart little girl and responds well to positive training. How often should we be doing appropriate length (15 mins or so) training sessions with her? She has access to a fill fenced half acre yard and a GSD older brother to play with, so she gets lots of exercise. Our objective at this point is improving recall, leash skills and things like leave it, drop it, off. I feel like she is not making enough progress.