Our main black water tank is 100 gallons, and takes about a week to fill when there are 6 or seven people on board. If just my wife and I are on board, we generally empty the holding tank every 2 weeks, when it is typically a bit over half full. If we wait 3 weeks, it will be pushing the red zone. Our experience is consistent with the assumptions that two adults flush 5 times per day each, and that each flush is 1/2 gallon.
Our grey water tank holds about 50 gallons, and has a float switch to automatically pump anytime new water is introduced. If we are anchored in a secluded cove, we will often turn the pump off, at least at slack tide. Without pumping, it takes 2-3 days for the two of us to fill the grey water tank. By the way, the kitchen, cockpit and flybridge sinks drain directly overboard, as does the washing machine.
this "5x per day at 1/2 gallon" set me to thinking
based on the reports of tank sizes and length between pumpouts reported in this thread
and adjusting for 2 people when more were stated
and assuming two people when no headcount was stated
and realizing that most of these are presented as off the cuff crude ballpark estimations from memory
and doing this off the cuff from my recliner, so I might make some big errors....
I've got
70 gallons/7 to 10 days = say 10 gallons per day = 2.5 gallons per person per day
40 gal/14 days = 2.9 gallons/day = 1.4 gallons per person per day
65g/14 days = 4g/d = 2 gallons per person per day
35G/7 to 10 days = say 4 G/d = 2 gallons per person per day
75/7 = 10.7g/d = 5.4 gallons per person per day
40/7 = 5.7 G/day = 2.9 gallons per person per day
40/14 = 2.9 g/day = 1.5 gallons per person per day
33/7 = 4.7 g/day = 2.4 gallons per person per day
so 2-1/2 gallons per person per day is probably a pretty good rule of thumb number....
for two people, 35 gallon is a 'one week tank'
with a treatment device it comes down to how long might you want to anchor out our transit through these no discharge zones...but perhaps the 35ish gallon tank might be ok in my thinking except maybe for doing the great loop where you'd be in the great lakes for some time
but without such a treatment device, I'm thinking 70ish gallon might be a minimum tank if you don't want to hit a marina every week.
