We have 2 40 gallon black tanks. The main bathroom gets used by all 4 of us (2 adults, 2 teens) and that black tank is emptied usually every 6 days (we camp seasonally). The half bath's black tank, really only used by the kids unless the main bathroom is occupied. That tank gets dumped every 3 weeks or so.
I have never actually measured the capacity, but our Class C Motorhome had a 32 gallon black tank and that was dumped every 5 days with all 4 of us using it, when we camped seasonally with it.
-Michael
