If a system is seen to be free of fouling from airborne particulates or process debris, then there is no set interval for a clean to be carried out.
However, HSE do recommend a precautionary approach and a schedule of planned shutdowns at pre-determined intervals. The system should be inspected, disinfected and (if required) cleaned if there is a significant change to the status of the system, such as:
immediately before the system is first commissioned
after any prolonged shutdown of a month or longer (in particular circumstances, a risk assessment may indicate the need for cleaning and disinfection after a period of less than one month)
if the tower or any part of the cooling system has been physically altered, e.g refurbishment or replacement of pumps, pipework or heat exchangers