Appliance figures are typical full-output ratings, not a specific model. Check the rating plate or manual for yours (it lists kW or g/h) and type it in. R380 is an example 9 kg refill price; change it for other cylinders.
How we work it out
LPG holds roughly 13.8 kWh of energy per kilogram. An appliance burning at 1 kW uses about 72 g of gas an hour, so a 9 kg cylinder lasts around 125 hours at 1 kW, or about 30 hours for a three-panel heater on full.
Real cylinders run a little shorter. In cold weather the gas vaporises more slowly near the end, and most people swap before the cylinder is completely empty. Weighing the cylinder tells you how much is left: subtract the empty (tare) weight stamped on the collar.
Related: heater size and running cost calculator, gas heater buying guide, skottel guide, patio heater guide.