I have a 4000 and the canisters hold 2 liters (each). I only use one burner at a time, cooking
for myself every morning and about 5 nights a week I get anything from 1 to 2 months out of a gallon. I don't often cook things that take longer than about 20 minutes at a time, so i think my usage is pretty low most weeks.
1 liter a week sounds about right for average use. But conserving fuel as much as possible by using things like a thermal cooker, pressure cooker, or just eating simple meals
and preventing evaporation with a some type of cover over the canisters are definitely good ideas.