The NPMA guide is a great tool, good inclusion Ehab. I would counter your comment about monthly visits, in North America, the "normal" tends to be weekly.
Tamale, it comes down to it being your plant, and your money, and as with everything, you should be seeking to get best value. As mentioned earlier, a professional (that meaning someone qualified to assess a food processing facility for the potential risks of pest activity given the geography, environment, product and facility itself) should provide their best plan based on the assessment. The main intent, needs to be pest exclusion, which is sometimes forgotten. If the exclusion practices are sound, then the failure indicators (think traps) will identify a potential weakness.
Like any other risk analysis....
Risk assessment = initial assessment
Monitoring = traps and site visits
Non-conformity = evidence of internal activity (the thresholds may differ by industry) or product at risk
Correction = how to react to a problem indicator
Corrective action = generally responding to technician visit recommendations
Re-assessment - the annual (or when necessary) review of the trends and previous risk assessment.