how often you run, what fuel you run, the type of oil you run, whether you run a vacuum pump or not, are just a few of the factors that influence how often your oil should be changed. But the number 1 factor in determining how often your oil should be changed is your conclusion based on the opinions of others that you have read or been told.
For me, 40 passes running Mobil 1 5W-30 in my 900+HP BBC running on gas.