I carry another small canister as back up as well.
Boiling water for five days for one man with one 8 oz. (actually weighs 13 oz. because empty canister weighs 5 oz.) is pushing it IMHO. I weigh my canisters after use and mark the weight on the bottom of the canister with a magic marker. For five days I would take a half-empty canister and a full canister.
If you don't have a half-empty canister then Chiro's idea of carrying a small canister is not a bad idea.