Oh, you'll know it's full when you can start to see the stanky water.
I can't tell you the last time that I looked at the idiot panel.
For our black tank, we wait until the toilet "burps" at us. This is approximately every 6-8 days. We then either empty the 1/2 bath toilet also OR do that every other time. Apparently the kids don't go nearly as frequent or as much as us adults.
For our gray tanks, we generally empty the shower every other day OR leave it open OR wait until the water backs up into the shower basin. Depending on our mood. The sink tank gets emptied periodically as we're emptying other tanks and rarely fills up.
We use the Geo method and that supposedly makes a difference in sensor cleanliness. But again- don't look at them so I don't know.