What is burning time for gas canister. Large cobbled area needs attention

A standard butane/propane cartridge (around 220–450g) will often give roughly 1–2 hours of intermittent use (on/off as you move around) at a moderate flame.

Running continuously on a high flame can reduce this noticeably (often closer to 45–60 minutes).

Because burn time depends on flame setting, type/size of canister and ambient temperature, it’s safest to plan on having more than one cartridge for a “large cobbled area”, especially if you’ll be running at a fairly strong flame to heat weeds between stones.