Here's an article that talks about the process of fat adaptation. https://medium.com/@davidludwigmd/adapting-to-fat-on-a-low-c... Note the graphs showing ketone levels continuing to rise through 38 days of complete fasting. If you are ingesting some carbohydrates or protein, this would slow the adaptation process beyond that.

There are lots of studies showing a performance decrease during within those early few weeks of the adapatation process, but they aren't really showing the fully adapted, optimized state.