This means you have to be in a facility, diagnosed and prepped for intervention within three hours, so ideally you need to be evaluated within an hour.
If you have any symptoms of stroke, even if they are brief and then disappear, get emergency care. Call an ambulance. It may save your life.
The American Heart Association
The National Institute of Neurological Disorders and Stroke http://www.ninds.nih.gov/disorders/tia/tia.htm