I'm a big fan of the Waterless wash and use it on my car regularly. My rule of thumb is to use waterless on dust/pollen and a full wash after much rain driving. I have coated my car using Adams' ceramic paint kit. That makes the waterless wash option far easier as the coating makes all washing safer. The method for using waterless wash should help figure out how dirty is too dirty as well. You just spray it on and drag the towel across the surface using little pressure. This means it isn't going to pull up road gunk allowed to dry on the paint after a rainy drive. However, the coating can help here too as driving in light rain will leave very little behind as the water just shoots off.
So yeah, not really that helpful I guess as it is an experience thing a bit and as Mathews31 indicated, depends on how much you've done to the finish already.
Also, I love the color of your car