I am not sure everyone is talking about the same cassette toilets.
The Thetford 403C holds 5 gallons of waste and you need to see a doctor if you fill it up in a day. We carry an extra cassette for 10 gallons total storage, which seems to last about a week or so.
Maybe the small ones only hold a couple of gallons?
I do agree that if you are boondocking for several weeks in some remote area you probably need to figure out a bigger storage system. We plan on using WAG bags if we get in a situation where we can't empty the cassettes for many days.