Every 6 months if you are being good, every 12 months if you are being ok. Anything over 12 months you don't deserve to have a 911
The milage doesn't count that much unless you are doing big milages. Short trips and not much driving is even worse for your oil as it can contain moisture in solution and never gets to burn it off. The water gets there from condensation.
Also change your gearbox oil at least once every two years - again cheap insurance.
The above info is just my opinion.
Bill '79SC