I go different periods of time and mileage depending on the vehicle and how it is driven. I run Mobil one in my 350 Tuned Port engine in my daily driver El Camino. It sees 60 miles a day, mostly highway, and is still pretty clean when I change it between 7500 and 10,000 miles, and does not use any oil. The original 305 went 280,000 miles without the benefit of synthetic oil for the first 180,000 miles ! When I pulled it apart, it was clean with no visible wear.
My Barracuda with 360 six pack , Mobil One oil, gets alot more contamination in the oil, and gets changed every year. It sees little mileage usually , but sees too many warm ups.