When they are ready
One of the most difficult tasks in the education of therapeutic play might be how to be able to use every moment to provide opportunities for progress. Progress in communication, in fantasy, in speech and in whatever is functional for the child.
One thing is for certain: Development only occurs when the children are ready to learn something new, and not when we want that to happen. In fact they will not learn at all, if we NEED them to learn in order to feel better ourselves! That puts pressure on them and will hold them back.