Johnny's home is m miles from school. Johnny runs to school at a speed of 8 mph and then walks home from school at half that speed. How much longer does it take Johnny to walk home than to run to school?

To run to school, Johnny travels m/8 miles, which takes m/8 / (8 m/h) = 1/64 h = 1.5 minutes.

To walk home, Johnny travels m/16 miles, which takes m/16 / (4 m/h) = 1/64 h = 1.5 minutes.

Thus, Johnny takes 1.5 - 1.5 = <<1.5-1.5=0>>0 minutes longer to walk home than to run to school. Answer: \boxed{0}.