I would go with at least 36 hours personally. With 24 hours your plant is only missing 1 cycle of 12 hours of light which might not be enough to freak it out all that bad, with 36 it is at least losing 2. When I go to do it I will let the light shut off like normal one night, and then harvest 2 mornings later (so 36 hours dark, with 2 light cycles lost).
24 hrs = 1 lost light cycle
36 hrs = 2
72 hrs = 6