I used to use an MSR XGK a lot, and I would estimate fuel more on the basis of time. For one stove, I used to get a minimum of two hours of full flame per liter of fuel. I would expect similar Primus models to use similar amounts. Obviously I could get more burn time from the fuel by turning down the flame, but that did not necessarily get better results from snow melting. The efficiency of snow melting will vary a lot depending on the water content of the snow. In other words, you will get faster results from slush than you will from powder. Also, the efficiency will vary depending on windscreens and stuff like that.
So, just stop and estimate how much time you expect to be burning the stove each day. I'm guessing at least an hour per day.