Exhaust systems should be inspected at least once a year as part of regular vehicle maintenance. However, the frequency can vary based on several factors such as the age of the vehicle, driving conditions, and manufacturer recommendations. For vehicles that are frequently driven in harsh conditions, such as extreme temperatures, high humidity, or on salted roads, more frequent inspections may be necessary, possibly every six months. 
Older vehicles, particularly those over five years old, may also require more frequent checks due to the increased likelihood of wear and tear. Additionally, if a driver notices any unusual noises, decreased fuel efficiency, or a change in exhaust emissions, an immediate inspection is advisable regardless of the last maintenance date.
Regular inspections help identify issues such as rust, leaks, or damage to components like the muffler, catalytic converter, and exhaust pipes. Addressing these problems early can prevent more significant damage and costly repairs. Moreover, a well-maintained exhaust system is crucial for optimal vehicle performance, fuel efficiency, and compliance with emission standards.
In summary, while an annual inspection is a general guideline, individual circumstances may necessitate more frequent checks to ensure the exhaust system's integrity and functionality.