Couldn't do it, but a roomate I had in college would leave her relaxer in for over an hour. She said "The relaxer isn't going to do anything, my hair just sits up and laughs at it." Her hair was, possibly 2 inches long, not cut into any style or anything. But she was right, the relaxer did not actually get her hair straight, but I always wonder if the hair becomes crinkly due to the damage, not because the relaxer didn't work. Anyway, she's natural now so it's all water under the bridge.
As for me, regular strength for 15 minutes max, or else I will lose clumps of hair and have bald spots.