For residential properties, your furnace should be cleaned as often as you feel it should. It is kind of like asking someone, how often should I sweep my floor. The answer varies greatly depending on use and a person’s personal cleaning standards.
Generally, you should clean your furnace and ducts after a major renovation when the furnace was running, soon after purchasing a home if it hasn’t been done in the past year, or every 1-2 years if you have bad allergies or asthma or have animals in the home that cause guests to have allergic reactions when visiting. For general house cleaning purposes and to reduce the amount of dust circulating throughout the home, it is generally recommended every 3-5 years but it is not required.
Many companies will recommend cleaning every year, but there is no set requirement for residential homes. There are only set standards in place for places like hospitals.
To read more about when you should have your system cleaned you may want to read what the EPA (environmental protection agency) recommends.
One thing that the EPA does not mention is that in Canada most homes cannot shut off the furnace for prolonged periods during home renovations which is why in Canada furnace and duct cleaning is a more regular occurrence than in the US.