It's almost always been zero minutes with me. I always wait until either the electricity suddenly goes off or I start hearing a roar before I turn chicken and rush straight for shelter, which is a safe room. Fortunately, that moment has yet to happen in the house I've lived in for the past 9 years. The last time it did happen I was in a house that didn't have a safe room. When the electricity suddenly went off, I finally got real scared. I quickly pulled the mattress off my bed and took it to the hall where I got down and covered myself with it. About 30 seconds later the loud roar of the tornado went over. Fortunately, little damage happened other than an uprooted tree that spread itself over the back yard, rather than on anyone's house. But several blocks away, where the center of the F3 tornado passed through, entire roofs were blown off.