Air conditioner cleaning

Dear Fellow Residents of Tokyo,

We got a flyer in our mail box today from Keiyo Gas, advertising their air conditioning cleaning service. We've been in our current apartment for about 18 months and although I've cleaned the removable filters and used the auto clean function, I assume the units should be professionally cleaned as well. If anyone has info/advice on the following questions I'd appreciate it:

1. How often should the units be cleaned professionally? (We are average users – not on all the time but used for heating and cooling).

2. Can anyone recommend a company from personal experience?

3. How much can I expect to pay per unit?

4. Any other tips?

Thank you in advance, fellow a/c users.