In general, AC cooling units need to be cleaned whenever they get dirty. Several factors affect how fast dirt accumulates in coils. The duration of the operation, of the space around the cooling units, and dust clogged filter are the major reasons that contribute to dirt build-up.
You should clean the coils a couple of times a year. However, in case of dirt may accumulate fast, you should perform a thorough cleaning of the cooling units using a cleaning solution. It is also important that you keep the area around the coils clean, and clear the clog from the dust filters regularly.