That's what I do, too--try it out on the trail, weighing the canister before and after. You can get a fair idea, though, by preparing a couple of backpacking meals on your patio at home, weighing the canister before and after.

My own findings are that with four people, boiling 1 quart of water three times per meal twice daily (once for hot cereal or a one-dish dinner, once for cocoa/tea, once for sanitizing dishes), a large MSR canister (8 oz. isobutane) lasts 2 breakfasts, 2 dinners. It will run out during the second boil for the 5th meal. (I really should get a 2-liter pot so only 2 boils are needed!) When I go out solo, I eat cold breakfasts so boil only 2 cups for dinner (one-dish dinner reconstituted in a freezer bag, plus tea) once a day. If I take one or two non-cook meals, I can go out for 9 to 10 days with one small MSR canister (4.1 oz. isobutane). If I'm going to be fishing, though, I take a large canister for the same period so I can fry the fish.

EDIT: A quart of water takes about 5 minutes to boil. I never turn the burner all the way up--it's more like halfway. This is using the Primus Micron which is comparable to the Pocket Rocket or Snow Peak Gigapower.

I can't understand the "40 minutes at high heat" scenario even if you're doing a lot of cooking from scratch. Normally items being cooked from scratch are brought to a boil but then simmered on as low heat as possible. Check out Sarbar's website (she's a regular contributor here) for lots of recipes, many of which use supermarket ingredients, that require only adding boiling water and sitting 15 minutes in a cozy to keep warm. This saves a lot of fuel, time, and pot-scrubbing. Most of us here use this method or some variation on it.

Please note that the canisters for the Pocket Rocket are isobutane, not propane. The Pocket Rocket is not designed for propane and the results could be more spectacular than you'd want!