i dont understand if you are asking what a damping factor is, or what THE damping factor is for a twisted wheel, as in a numerical value. i cant give you a number, but ill tell you what a damping factor is in case you dont know....

a damping ratio is a unitless measure of a damped oscillation. when a wheel absorbs an impact/impulse, it ocsillates. a damped oscillation is a oscillation in which the amplitude decreases over time. when a system with a high damping ratio receives an impulse the amplitude decreases over a longer amount of time than a system with a low damping ratio.

citing gerd schraner from his book "the art of wheelbuilding" --

the twisting of spokes, instead of classic crossing work is a waste of time for road use. tests have shown that it results in a very unstable wheel. the lateral stability is about the same as a normally spoked wheel, but radially the wheel reacts so sluggishly that encounters with momentary radial overloads the highly praised damping effect is delayed. ... twisted spoking patterns are an additional mechanical disadvantage resulting from the extreme angle of the spoke directly at the nipple. (schraner, p. 60)