Most often, the judge will rule on the spot, because the whole point of a *preliminary* injunction is that time is of the essence: damage or financial losses of some sort or another are happening or may happen if the injunction is not granted, so delaying the ruling extends or increases the losses. (Or conversely: if  there is already a temporary restraining order in place, delaying a ruling could cost the other party money or otherwise injure them, if the restraining order is not appropriate and they should not be restrained.)However, the law does not mandate an on-the-spot ruling, and if there is complex evidence to review, or legal research that needs to be done, the judge has the right to reserve the ruling an issue it later.