How do I know when to place my next order for my gas appliance?

Any gas appliance will have a power rating (in BTUs or KWs) stated on its data badge. This shows how many grams (or kilograms) of gas per hour is used. So it will be possible to calculate approximately how long an cylinder will last on a particular appliance, depending on the heat setting is selected.

A typical cabinet heater for example will take approximately 118 hours to consume 13kgs of gas if operated continuously on its lowest setting.

Depending on the appliance there are also a number of ways to detect when gas is getting low in the cylinder as follows:
For high heat output appliances (like industrial forced air heaters) the normal operation and efficiency of the appliance will begin to diminish and the burner sound drop. A band of frosting or heavy condensation may also appear on the cylinder(s)

For butane appliances (like a infrared cabinet heater) the flame intensity will drop on the ceramic plaques and the pilot burner flame will begin to ‘lift’, indicating a weak air gas mixture, and become noisy in operation.

If you are a regular gas cylinder user then a pattern of usage would be a guide for when a replacement is due, e.g. a cycle of a cylinder every 3 or 4 weeks.