We have a 44 gallon black tank. There's 2 of us. Last summer, we boondocked in the middle of nowhere for a week. Husband peed outside during the day, inside at night, I used it throughout the day/night. It was pretty close to full at the end of the 7 days. But, not to be TMI or anything, but he has a stomach disease so #2 was done inside and he uses extra flush water for that.
In our old camper, which was around 30 gallons, we boondocked for about 5 days, and used the facilities inside the music festival we were at, when we were in there, about 5-7 hours a day, but used our bathroom otherwise, and we ended up about 3/4 full by the end of the 5 days.
Both of these were in hot climates where we drank a lot of water and alcohol. I'd guess we could last a bit longer in a cooler climate since we wouldn't be drinking as much (water anyway haha).
All that to say, I think if you just reserve it for use at night only, you'll be fine.
