Quite a few variables to your question, but here is a general answer.
The key to selecting the correct service interval is to balance the target pest/conducive conditions/product residual equation. First, some pests require more frequent treatment than others. Second, some homes have more conducive condtions than others (i.e. location, construction type, age, condition). Finally, different product choices require different application intervals due to a loss of residual effect or palatablility in the case of bait products. Without knowing your specifics, a general pest treatment for 'bugs' every 2-3 months should be ok to the exterior of the home. Keep in mind that conducive conditions may require more frequent inspection, but not more frequent application. Also certain pests may require multiple applications of different materials on different intervals (i.e. rodents or 'crazy' ants). This information is based on my field experience as a pest control operator in Houston, Texas.