Of course all our needs and wants are different, but you make a good point, Nubbie. The topper will affect the amount of juice as well as our habits. Even some of the smaller toppers will use more juice, for example, any of the dual coil units. It's how they deliver more vapor - by boiling more juice!
I think what OP was wondering was how much juice to have on hand. I know when I started, that and concerns about other backups were high on my mind. While situated in an area where I do have several B&Ms relatively close, they're still far enough away to make the trip non-trivial. So having enough stock on hand to avoid needless trips was important. Others, might be online is their only source, so having a clue on when and how much to order - and then stalk the postperson! - it's likely more important.