It's a relative thing. You need an idea of how your stove really burns fuel for the kind of "cooking" you do, so it's no good to just do this once.

Weigh the canister when you get it. Write it on the top of the can in Sharpie Marker.

When you get back from a trip, weigh it again, and note how many boils you did.

Next trip do the same. You start to get an idea of how much fuel you're using per boil.

When empty, measure the can - gives you an idea of what an empty really weighs for next time.

Variables include temperatures (very cold operating temps = less efficiency) and amount of water boiled.

I can tell you with some certainty that the way to get a solid boil more quickly is to place the canister in a dish of water (not warm or hot, just water). Worked great last trip when it was 30F in the morning and the stove was being wimpy about heating the water.