RV holding tank capacity

After 7 years of TM ownership I've gotten pretty accurate at judging the capacity of the Thetford. (About 8-9 "user days" or a 4 day weekend for DW and I before we need to pull the valve and use the super secret emergency reserve.)

My question is, how does this correspond to using a standard RV flush toilet and black water holding tank? Put otherwise, how many user days are there per 10 gallons of black water holding tank capacity? Thanks! -camp2canoe