I don't think you ever stop copper fouling. Copper fouling is just a fact of life with a piece of copper going 2000 mph down your barrel. Now, you just need to determine at what round count does it start affecting accuracy.
With 300 rounds the bore should be "deburred" (or whatever they call it) so just keep shooting until you see accuracy suffer. I'm at 400+ round without cleaning and haven't see accuracy degrade. I've got 2100 round through my CL barrel without cleaning and it is still shooting fine, but CL is a different animal.