Douglas fir 2x4x8 lumber, 2" being top/bottom, 4" side, 8' wide, how much weight can is hold from the middle?

Say a 200 pound man stand in the middle of the 8 foot lumber, how many hour would it take before the lumber crack? What if two 200 pound (400lb) person stand in the middle of the lumber, will it crack? How much weight do you think this 8 foot lumber can withstand if the pressure where right in the middle? What about a 2x6x8? Thank you very much.