This is one of the most difficult questions to answer because there are so many factors at play: the climate zone the house is in; the size of the space to be heated; the thermal qualities of the house (how well it retains heat and whether it has good passive heating); the temperature you find comfortable; and how often you are at home.
However, a starting place is the usage assumption we referenced in our cost comparison post. That is, using an 8kw heater (either the Malu, the Lia or the Nina) for 5 hours a day, two and a half hours each on a high and a low setting. The Australian standards assume 100 heating days a year and this would work out to a yearly consumption of 600kg or 40 bags (a bag of pellets from EnviroHeat Australia costs $7.74 for a customer who has bought a heater off us, excluding delivery). We would consider this moderate use in a mild climate. In colder climates where the number of heating days is closer to 170 and outside air temperatures are lower, annual consumption will be substantially more.
Alternatively, if you have ever used a log burner and if you are heating for the same amount of time and to the same room temperature, you should expect to consume roughly half to a third the tonnage of pellets as you do firewood. The average household in WA that operates a wood fire consumes 2.5 tonnes of firewood; this would translate to about 1 tonne of pellets.