I am a member of the National Association of Chimney Sweeps which means:
I issue an official NACS certificate for each chimney swept which will confirm it has been done in accordance with the NACS code of practice and also indicate a recommended date for subsequent sweeping, together with details of any defects found.
You can visit the official NACS webpage by clicking here.
Although it will vary depending on factors such as frequency of use and condition of the flue, as a general guideline chimneys should be swept as follows:
For maximum efficiency and cleanliness each chimney is swept using a full brush and vacuuming system.
Regular sweeping is essential, not just for optimum efficiency of your flue, but also to prevent more serious incidents such as chimney fires and carbon monoxide poisoning.