Without knowing the actual dosage given, it is impossible to give you an exacting time frame. If he was given the
Risperdal Consta injection, the effects would persist beyond one month. This would seem highly unlikely.
In my opinion, this toxidrome will resolve without incident in a week or less. If there are no signs of improvement soon, definitely head back for the ER and demand some answers. Keep an eye on temperature and pulse, as those are two good indicators for NMS.