How many days does it take the acid rain to clear up?

It is not a matter of days because it is caused by humans. If acid rain was a natural process, then nature would take care of it when it was needed. But since acid rain is created by humans, nature has no correcting mechanism for it, so it will remain unchecked until we humans do something about it. And, unfortunatly, that will probably take more than days, weeks, or months. Maybe years.