Air Conditioners

Hiya all,
I have a query about Air conditioners. With the hot season here some of the ACs in my house are on nearly 24hours! I understand that they are merely recycling the air in the house (plus cooling it).

My question is, how often should we be opening the windows and letting in the (hot) fresh (?) air in to 'change' the air in the house? Back home we don't tend to use the ACs that often, but rely on the afternoon sea breeze to come.

Also, how often should AC filters be cleaned? Can this be done at home (by ourselves) or should this be done professionally?

Thanks