If you have a Gas AGA then you should have it serviced regularly once every 12 months. As it is in constant use when on you will find that your AGA will need certain parts like the thermocouple or the gas cock replacing regularly.
As for electric you should have them serviced regularly but not as often. This ensures that your AGA is operating at its full potential and any faults found can be dealt with to prevent any issues that may occur.