there is no set time interval for cleaning. The cleaning has been put by diagnostic technicians.
Cleaning a hot wire MAF is done when the hot wires become contaminated with oil or dirt - This is seen via the MAF flow PID in OBD diagnosis - if the sensor is not registering correct values then the MAF sensor is suspect (please note the suspect).
The MAF sensor can be registering a fault even when there is a leak in the air tube after the MAF or even the intake manifold gasket or even throttle body gasket.
Diagnose it correctly first then proceed to clean. Diagnosis also encompasses reading the fuel trims (where applicable). Dont just make it a habit of shooting cleaners at the MAF every 6 months because you think its a good idea, It can destroy it too.
Also note that not all types of MAF can be cleaned, e.g. hot film maf or karmann vortex type should never be cleaned with anything. It kills them dead immediately. - Use a non residue degreaser to clean it - make sure that degreaser does not harm the nylon housing of the MAF either.