I think sooner than 6 months in-between is rare. Following my last treatment which ended in December 07, I had a PET/CT in April of this year. It was clear. I started having some chest/back pain two months later in June - just upon deep breathing. They sent me for a chest x-ray but it was clear. The pain persisted and had gotten worse, so last week they sent me for a PET/CT. It indicated metastasis to both lungs with PET/CT correlation on the findings. I've had a couple of people ask me why they didn't send me sooner for a PET/CT but I'd just had a clear one in April, so I think it was prudent to wait. I had no other symptoms except the chest pain, which wasn't constant. I'm not sure if it's different for CT alone as opposed to PET/CT fusion, but I've not heard of PET/CT being more often than 6 months unless they're trying to verify that a previous result was a false positive from healing tissue, etc.
-Steve