Just like any other element of your home, your computer will be susceptible to household pollutants. If you have a furry friend who sheds a lot of hair, this may mean you have to clean your computer inside more often that those without fur-babies. Furthermore, smoking around your PC will mean that a more frequent cleaning will be beneficial to your machine, every 3-6 months is a good benchmark.
Even if it might seem appealing to hide your PC under a desk on the floor, it will be more susceptible to dust and carpet fibres here. If you can, putting it on the desk or a shelf will help keep it dust-free for longer. If you follow this advice, a yearly clean is usually enough. However, if your home gets dusty quickly, you might want to do a bi-annual clean instead.