Lets say you live in a known risk area for termites (count the advertisements in the telephone directory).  It is normal for an inspection to be recommended to be done not less than once a year.  Some people tend to stretch that out a bit.  If you do go for less-frequent inspections it will almost certainly impact on any insurance or warranty you may have covering the termite risks.  Read the contracts.

If you live in a high risk area, have had termites or have a property that is poorly protected, then more frequent inspections are highly desireable. 

A good pest manager will often suggest inspections a few weeks or months after any works and then a few months later.

It is all a matter of balancing the risk of undetected damage with the cost of more frequent inspection.  In general, go for once a year unless advised otherwise.