You really need to do some testing. The results may depend on how good the fuel is that you're making.
I know that with waste veggie oil (WVO), and presumably with virgin veggie oil (VVO or SVO), the normal oil change interval is cut by roughly 2/3. So if the normal OCI in a VW TDI is 10k miles, with WVO or SVO the interval should be roughly 3k miles.
If your biodiesel is extremely high quality, then maybe you don't need to cut the interval at all. It all depends on the quality of injection and completeness of combustion -- you do not want any raw biodiesel being scraped off the cylinder walls and ending up in the crankcase. Diesel engine oil is designed to keep working if there is some diesel fuel contamination, but not with veggie oil or biodiesel contamination.