I use whichever is more economical. I adhere to a 6 month rule. Even if it's synthetic, I change it at 6 months. If it's cheaper for me to run synthetic, I run synthetic. My time is worth something also, so if I can go less often, I'll spend $20-$30 a year more for synthetic.
My grandmother drives 3000 miles every 4-6 months, so her car gets changes 2-3 times per year. It would cost more to keep synthetic in her car, which used to be mine and had synthetic while I owned it, because I put 30,000 miles per year on it. it cost $25 per mineral change, and 60$ per synthetic change. It cost me $50-75 vs $120 per year for grandma. If I had used mineral when I drove it, it would have cost me $250 per year vs. $180 with synthetic.
Presently I put about the same 30,000 per year on the Impala, basing that on 15,000 for the first 6 months. It would cost me $250 year for mineral. It will cost me $240 with the 8000 mile synthetic change. I could probably push it further than the olds, since it has an oil cooler, but for now I'll stick with 8000.