Welcome aboard mikebmaxum3000!!!
If its a manual pump, then one way to tell when it's full is the down-stroke will begin giving much more resistance. AND, it'll start coming out the vent tube.
Technically, contents shouldn't e allowed to sit for more than 5 days in the blackwater tank. If you're a weekend warrior like most of us, then the easiest thing to do is get pumped out at the end of the day on Sunday. This allows you to start every weekend with an empty head (no worries and no chores to start the weekend). It also significantly reduces head odor.
Depending on the number of people and how conservative you are with water, you will eventually figure out about how many consecutive days you can go before needing a pumpout. Here are my rough numbers for 2 people.
I estimate we fill aprox. 6+ gallons daily (2 adults). We are very conservative with flushing, (we drink a lot of beer).
Tank Size / Pump Out Frequency
10 gal. = daily
13 = every other day.
20 = every 3 days