There's no straight answer to this question as it depends on a number of factors, these include:
The way to tell if your system needs cleaning is to have a look at the grills/registers. Are they dusty or is there accumulated dirt on them?
Shine a flash light through the grill/register, do the ducts behind the grill/register look dirty?
If so, then you should get your system cleaned.
As a general rule of thumb, we say every 5 years. If you live in a dusty place or close to a freeway then maybe as soon as 3 years. If you change the filters regularly and you live in a clean environment then it could be fine for 7 years.
Another factor could be if you or anyone in the house has allergies.
If you move into a previously owned house you should get the system cleaned. Since dead skin makes up most of what dust is made from then cleaning the system makes hygienic sense and helps anyone with allergies.
If you have any further questions about this please call Nancy at 1 800 358-3828 and she'll glad to help.