Look at her round off, a round off has similar attributes to a back handspring. The gymnasts needs to be able to keep her body tight and are flight both onto and off of her hands, block through her shoulders and rebound. If she can do a round off with good tension, speed and power that's often a good indication of a solid BHS.
The speed at which she gets the skill will depend on a number of factors - confidence (is she fearful with other skills?), ability to adapt and change what her body is doing. Based on her coaching and the quality of the way she is coached. The latter will play a huge determining role in the speed at which she gets it - effective drills, a coach who understand the body concept and consistent training usually bring the skill quite quickly.