Empirical evidence
I have used Q-Wiz discs for years, off and on. I carry one in my case and maybe once every few weeks I will be in the middle of a match and feel a shaft getting a little sticky. In this situation, I may whip out a Q-Wiz and stroke the shaft may 6 times in long, fairly light, strokes. Then burning with the other side.
Just now, to get some empirical evidence, I took a dowel rod and used a Q-Wiz for 60 very firm strokes, cleaning the Q-Wiz after every 20 strokes. My digital caliper reveal that the Q-Wiz took off .01 to .02 millimeters in the diameter of the dowel rod. Now the dowel rod may be softer than a typical cue shaft, I don't know.
So I'm guessing that 8-10 normal uses of the Q-Wiz may reduce shaft diameter by .01 mm at most. If you use one every day, and press down pretty hard, you may be taking the shaft down at the rate of .01 mm per week or half a millimeter per year. It wouldn't surprise me if a shaft went from 12.75 mm down to 12.50 mm within a year of Q-Wiz use at 2-3 times per week.