It all depends on use. We just were out for 3 days with our rig (seismic 3712) and did not even fill the gray tanks to 1/3. Now, we used a combination of paper and regular plates and silverware so we did not have to do dishes as much, and we did not take showers every day. You have to ration the fresh water that goes into those tanks. That means not as much dishwashing, not as many showers, and the showers you do take will not have the water running continuously. For us, it is my wife and I and four kids, and 1 week would be no problem. If there were only two of us we could go for two weeks without much issue.
2000 Ford 4x4 Crewcab Dually 7.3 PSD
2015 Seismic 3712
2013 Jay Flight 32BHDS (sold)
2009 Jay Flight 31BHS (sold)
2005 Jay Flight 27BH (sold) - probably my favorite