Taking into account that loss of energy is NOT taken into account with this calculation (e.g. friction)
a screw is just a variation of a simple machine so the mechanical advantage (MA)= circumference of the screw over the pitch. I don't know realistic numbers off the top of my head but say your MA is 15 so it would take 1 pound of force to lift the platform. But obviously you're working with torque not force, but torque is just force x distance so the torque depends the diameter of nut.
Sorry, perhaps someone can give you some practical numbers from experience, but here's something to start with I guess, but really if you had a fine enough pitch (and little to no friction) you could lift the thing with the smallest of motors.