As others have noted, the sensors are widely known to be more or less useless. The gray tank is designed to overflow into the shower when full, the black tank will be truly full when you can't flush the toilet.
How fast they fill up is so widely variable that estimates from other peoples' data will be equally useless. Just as an example, we normally dump the tanks every 3 or 4 days whether they need it or not. Our small rig has a 15 gallon black and a 25 gallon gray tank. We've gotten 6 days out of that gray tank, and we had our daughter visiting us once, and she volunteered to do the dishes after dinner, and filled that gray tank in a matter of about an hour, bless her heart. In her defense, she knew we were on city water, and thought that the sewer line was left open.
Everybody I know requires several trips and a certain amount of experience to figure out how often they need to dump those tanks. Good luck, and camp more/worry less.
