> I imagine there's quite a large variation in how long a canister lasts
> between say 10000 feet and sea level.
No, altitude will have very little direct effect on the life of a canister. The canister supplies energy, and how much energy is in the canister is unrelated to altitude.
However, because the ambient temperature drops as you go up in altitude, you will find yourself using a little more fuel to get water up to 80 C. On the other hand, water boils at a lower temperature at higher altitudes, so you will need a little less fuel to go from 80 C to boiling. Ho hum ... :-)
Anyhow, most of that is irrelevant. What really influences practical fuel consumption is wind, use-of-windscreen, heating rate, lid-on-pot, and other things like that. They are far more significant, to the point of being totally dominant.