Back years ago when we had a popup I installed a Thetford 585 electric flush porta potti which as I recall had a 5 gal waste tank ... our family of four could easily get through a typical 2 nt weekend without me having to empty the waste tank until we got home, if it was a 3 nt long weekend chances were I'd have to do it at the campground dump station on the last day. Based on that I calculated we were generating ~ 0.5 gal of waste per day per person so in theory a 30 gal waste tank ought to serve the needs of just two people for up to a month. Even if one were to double that waste rate to 1 gal per day per person that's still up to two weeks of use for two people from a 30 gal tank. Since I service my tanks with a FloJet waste macerator I have the means to offload the contents of the black tank while still camping if necessary but now with just the two of us it's rare I'd ever have to.