It really depends on how efficient your stove and pot are, and how aggressively you boil your water. I find that getting water just barely to a boil is plenty to re-hydrate my freeze-dried meals in the advertised time. If you really get your water to a rolling boil each time, then you'll use more fuel. I use an Olicamp XTS pot and a Soto Windmaster stove that I run on low flame until the water just starts to boil, and can get over 30 such boils to an 8 oz can of fuel.
Having said that, I always carry a small backup canister of fuel just because I'm always thinking of the rare case where a canister leaks and leaves a guy with no fuel.
