Read the owners manual. As stated above, Porsche recommends 1/year or 15K miles. I bet there is an asterisk in there somewhere about "conditions may vary". If you live in a dusty area.. if you have short frequent trips that don't let the oil get up to temp.. if you tow with your car.. if you "race" your car... essentially, anything that may cause stress to the motor and accelerate the break down of the oils viscosity and protective features.
So.. how frequent? That in the end is up to you. Do you do a lot of slow dtop/go driving? do you race the car? do you never let it get up to operating temp? If so, more frequent changes are better. If you do nothing but "easy" highway miles for 30 or more minutes, longer intervals can be used.
To me, 3K miles is pretty frequent. I prefer somewhere between 5K-7.5K miles between changes (or at least once a year).
Oil is still cheaper than a new motor. You might also want to learn how to do this simple DIY item yourself. No matter who does it, get QUALITY oil and new filter everytime. Oil will still cost you about $60-75 for synthetic. Filter is about $10-15. Learning more about your car: Priceless
