Grow light question for bonsai?

I have a youngish brush cherry tree. It has been losing leaves and small limbs recently, so I'm going to go out and buy a grow light. About how many minutes a day should it get time with the light? Most of the limbs that have been lost are the ones that don't face my window. I've tried rotating it so that the whole tree could get the full effect of the sun, but then the healthy leaves die.