In fact, you should be doing your own checks more frequently - at least once every three months. Walk around the foundation, looking for mud trails, rotted wood and moisture problems. Rainstorms are a good time to crawl into the attic with a flashlight and check for leaks there. Most pest control companies include an annual inspection with the contract and should be willing to come out more often if you suspect problems. After treatment, pest-control operators usually do an annual inspection and return if the homeowner suspects termites.
Every situation is different.
Get four or five professional pest control companies to inspect your house and supply you with a detailed bid in writing. If they won't put it in writing, you don't want to deal with them. Most companies offer free inspections. Get a list of recent references in your area and check them. Call the Better Business Bureau to check for any unresolved complaints against the company.