How many miles between oil changes?

[GerryAllen]

I am new to the Model A world. I use my car as a daily driver, I put on about 600-700 miles a month, mostly at 35- 40 MPH, paved roads no hills to climb as such. My car is a standard Model A, no oil filter, using Chevron 15-40 detergent weight oil.
I know the original owners manual states change your oil every 500 miles. With todays detergent oils, multi grade, better road conditions, at what mileage do you change your oil?
Gerry Allen, Birch Bay WA