How many days to fill gray and black holding tanks???

...How much beer do you drink?

Seriously though, it all depends on size of tanks and how many people are using the coach. We get 3-4 days, two adults, 40 gal's each tank. If beer is being consumed in mass quantities, I use public facilities if available and clean for the returning of said beer to prevent too much usage of the black tank, but I have never filled my black tank.

You can help keep the black tank sensors working by flushing your tank every time you dump, not always possible though.