The cassette has ...20? liters. You really wouldn't want more, as that thing gets heavy. Which is the main reason why we usually dump it daily - half a cassette is much easier to handle. Plus, you are always good to go for the next two days, no matter what. For the same reason we try to keep the grey tank pretty empty at all times. The grey tank in our Arrow is quite small... we shower inside, wash our dishes, etc. and we do not waste water - but it will last little more than 2 days.
The fresh water tank is good for up to a week.
One advantage of the Thetford cassette in the Northstar is the fact that it has its own rinse water tank. You can fill it with antifreeze and use the toilet without dewinterizing the fresh water system.