It's not an absurd question. The threshold value is the one that breaks surface tension and effectively pulls waste away from the body. It will be more than a few hundredths g but less than 1g.

Unfortunately we have basically no data on the effects of partial gravity, in this context or any other. We can try flying partial-gravity parabolas in aircraft and simulate a Martian toilet the same way they tested the design for Skylab; I don't think this experiment has been done.