The number of hours in the day patching needs to be done has no consensus however with the time needed based on the severity of amblyopia
in the eye. There is a chance of the other eye becoming 'lazy' but that is why routine follow up is needed. The other eye vision is always noted to avoid this eventuality. At home as well, you need to observe your grand daughter when the other eye is open to make sure she has comfortable vision in both eyes and there is no deviation of the eye position. Along with patching, the child is made to perform certain tasks to develop binocular vision
and depth perception which is probably explained to you by your treating doctor. Patching during sleep does not help since both there is no visual stimulus.
How long you patch the eye will reduce as she shows improvement. It is difficult but has to be done and will be terminated when benefit is reached.