The 50 mph for the belt is sort of arbitrary, but assuming the plane's normal takeoff speed is similar, for the plane to lift off the wheels would have to spin at the same speed as if the plane touched down at twice normal takeoff/landing speed. While airplane tires are intended to have considerable safety margin, the real question is whether the tires would explode at the overspeed that might result - i.e. could the plane land safely after taking off from the conveyer.

Beginner pilots are assured that "no matter what fails, the reserve features will carry you to the point of impact," so it's probably not too important a question.

John