Our grey tank usually lasts about 3 good, long showers. Long meaning in the shower until the hot water gets cold. Or, it will last 4-5 quick showers. Our sinks go to a galley tank, they usually fill after doing dishes/brushing teeth 3 days/nights.
One comment I had about it showing how full each of your tanks are - we were told from the beginning not to believe what our panel told us, especially with the black tank. Matter could get in the way of the sensor, giving a false read to how 'full' it actually is. Also, we were told not to dump our black until it is full because if 'matter' sits in it but the liquid isn't that high in it and the tank isn't full, when you go to dump, only the liquid will come out, there won't be enough liquid to carry all the 'matter' out. Also, the tank's highest point is directly under where your toilet is, and then it declines down to where it dumps, so it is at an angle. The 'matter' just sits at the top, while the liquid drains down to the bottom where you open it to drain. So if there isn't enough liquid to reach the top, then you have the same problem and the 'matter' just sits there. If you do want to dump it that often, then I would flush it that often because you probably aren't dumping everything at 1/3 full. Just my .02 cents.
