If you have annual maintenance contract with Aircon services, then the technicians would anyways schedule you a visit to check your air conditioner every three to four months. However, if you do not have an annual maintenance contract signed, then you need to be aware when was the last servicing done and how much time has passed by since the last servicing. Keeping a check on this is important, because checking the filters and whether the compressor is working properly or not decides the longevity of your air conditioner.
If your air conditioner is giving our stale and pale odor or making strange noises, then it is time to call for Aircon Services. Also, if your air conditioner is not working or there is warm air coming out of the vent periodically, then it is time to call for Aircon services.
Other indicator that you need air conditioner servicing is that the energy bills are coming too high suddenly. Then in that case you need to call Aircon Services immediately, as the compressor might be overloaded. If that happens, there is a chance of a short circuit and your compressor might stop functioning completely, which can be really expensive to replace and might even need you to buy a new air conditioner.